J Vis

https://doi.org/10.1007/s12650-018-0493-3

R E G UL A R P A P E R

S. Mohamed Illyas • B. R. Ramesh Bapu • V. Venkata Subba Rao

Heat transfer and flow visualization of swirling

impinging jet on flat surface using helicoid inserts

Received: 10 October 2017 / Revised: 17 March 2018 / Accepted: 3 April 2018

 The Visualization Society of Japan 2018

Abstract The heat transfer and flow structure of swirling impinging jet on a flat surface with helicoid
inserts are experimentally and numerically analyzed. The study is focused on flow dynamics intended to
describe the influence of swirl on the mechanics of impingement by varying the number of helicoid surfaces
named as single, double, and triple helicoid inserts. A helicoid surface may be considered as having an
infinite number of adjacent helical curves that rotates symmetrically about Z axis. The thermochromic liquid
sheet and oil film technique are used to visualize the heat transfer characteristics and flow structure on the
impinging surface, respectively. The numerical analysis is carried out for Swirl number Sw = 0.75 and
Reynolds number value of 23,000 and for jet exit to impinging surface distance of H/D = 1, 2, 3, 4 using
CFD. The flow characteristics of swirling jet are also compared with circular impinging jet. The flow
characteristics are presented in terms of axial velocity variation and the distribution of vorticity and velocity
vectors are also visualized. In addition turbulent statistics are also presented. The axial component of
velocity of jet leaving triple helicoid at the stagnation region is relatively lower than single and double
helicoids due to the presence of axial recirculation zones and the tangential velocity component of triple
helicoid is higher in the region which corresponds to radial distance r/D = 0–0.4, 0–0.8, and 0–1.4 at H/
D = 1, 2, and 3, respectively, compared with single and double helicoids. The axial velocity component
exhibits flat profile for the single and double helicoids at increased H/D distances (H/D = 4). The vorticity
distribution is relatively more intense for triple helicoid at the downstream of jet near the wall jet region
causing it to entrain more ambient air compared with single and double helicoids.

Keywords Heat transfer  Helicoid insert  Swirling jet  CFD  Thermochromic liquid crystal

List of symbols

A Area of stainless steel foil, m2

D Inner diameter of jet exit pipe (outer diameter of vane), m
d Vane hub diameter, m
H Jet exit to impinging surface distance, m
H/D Dimensionless distance of jet exit to impinging surface
h Convective heat transfer coefficient, W/m2 K
kair Thermal conductivity of air, W/m K
Nu Nusselt number, dimensionless = khD air

S. Mohamed Illyas (&)  V. Venkata Subba Rao

Department of Mechanical Engineering, Jawaharlal Nehru Technological University, Kakinada 533003, India
E-mail: illyas1978@gmail.com

B. R. Ramesh Bapu
Department of Mechanical Engineering, Chennai Institute of Technology, Chennai 600069, India
 S. Mohamed Illyas et al.

U0 D
Re Reynolds number, dimensionless = c
r Radial distance on the impinging surface, m
r/D Dimensionless radial distance on the impinging surface
 
d 3
2 1ðDÞ
Sw Swirl number, Dimensionless = 3 2 tan h
1ðDd Þ
Sh Single helicoid
Dh Double helicoid
Th Triple helicoid
Ts Local impinging surface temperature, K
Tj Jet exit temperature, K
ur’ Root mean square of radial velocity fluctuations, m/s
U0 Mean velocity of air in the smooth pipe, m/s
c Kinematic viscosity of air, m2/s
h Helicoid vane angle, degrees
r Standard deviation of Nusselt number over the impinging surface
r Normalized standard deviation

1 Introduction

Impinging jets are widely used in the variety of engineering applications such as cooling of electronic
components, glass tempering, drying of papers, and food products due to their prevailing nature of affecting
the transport properties. The transport properties over an impinging surface can be better controlled by
vectoring the jets [Laschefski et al. (1995) and Page (1991)]. The heat transfer performance of impinging
jets is the subject of importance for many studies both experimental and numerical in the recent years. The
earlier studies by Mahmood (1980) Viskanta (1993) and Huang and El-Genk (1998) revealed the funda-
mentals lying in the flow structure of impinging jets. According to Viskanta (1993), the shear driven free jet
at nozzle exit consists of potential core where its velocity remains uniform. The potential core length
extends up to 6 to 7 nozzle diameter from jet exit (Livingood and Hrycak (1973)), and depends on the initial
velocity profile and the intensity of turbulence at jet exit (Viskanta (1993)). In the free jet region, the
emerging flow is characterized by a developing zone where the axial velocity decays by large shear stresses
at jet edges. Further downstream, the flow enters developed zone where the jet broadens and decay of axial
velocity is linear and eventually the jet enters the impingement zone (stagnation region) and turns radially.
The jet spreads radially in the wall jet region and decelerates with increasing distance from stagnation point.
Alekseenko and Bilsky (2007) conducted experimental study on circular and swirling impinging jets with
swirl number varying from 0 to 1 for Re = 8900 at H/D = 3. They analyzed the spatial distribution of mean
velocity and reported that vortex breakdown was clearly observed at Sw = 0.7 and 1 leading to intense
turbulence in the initial region of jet. The study by Fenot et al. (2015) examined the flow and heat transfer
characteristics on multi-channel impinging jet of round and swirling nozzles with straight line structure
(Sw = 0) and with curved structure (Sw = 0.26) for Re = 23,000–33,000 at H/D = 1–6. The study reported
that the presence of inner shear layer is observed for the swirling jet between the recirculation zone and main
flow and it disappears with increasing H/D distance. Wannassi and Monnoyer (2015) have examined the
flow and heat transfer characteristics of array of circular and swirling impinging jets (swirl angle h = 20,
30, 45) with nozzle to plate distance of H/D = 4 and jet-to-jet distance of s/D = 5. They reported that the
jet flow intensity relies on swirl angle and swirl motion and dissipates rapidly in the circumferential
direction causing less intense axial component to participate in the jet interference at the impingement
resulting in reduced upward flow rate. The numerical study by Herrada and Pino Del (2009) has explored the
flow characteristics of non-swirling (Sw = 0) and swirling impinging jets (Sw = 0.1–0.6) for Re = 100–500
and jet height of H/R = 10 and 16. The study revealed that vortex break down occurs for the case of
Sw [ Sw* (Sw* = critical swirl number), and it is initially steady and becomes unsteady when the swirl
intensity increases. Fairweather and Hargrave (2002) have examined the mean and fluctuating velocities of
turbulent impinging jet (Re = 18,800 and H/D = 0.2–1.9) using the PIV technique and illustrated the
influence of recirculation zone on the development of wall jet. The influence of nozzle exit profile and
varying exit geometries on the development of impinging jet have been examined by many authors both
experimentally [Fairweather and Hargrave (2002); Zhuyun and Hangan (2008); Deo et al. (2007); Ferdman
Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

et al. (2000); Suresh and Garimella (1996); Nathan and Mi (2010); Xu and Antonia (2002); Obot et al.
(1984) and numerically (Faghani et al. (2010); Shuja et al. (2005)] but limited to non-swirling jet.
While the upstream inflow conditions of swirling jet with the imposition of swirl potentially affect
development of jet and the wall heat transfer, the inherent physics involved in the flow structure are yet to be
addressed in the significant number of heat transfer studies [Ianiro and Cardone (2012); Yuan et al. (2006);
Lee et al. (2002); Bakirci and Bilen (2007); Eiamsa-ard et al. (2015); Yan et al. (2004); Cafiero et al. (2014);
Chigier and Chervinsky (1967); Rodrigueza and El-Genk (2010)]. For this reason in the present work, flow
and heat transfer characteristics of impinging jet have been studied with helicoid inserts of varying number
of vanes. The study encompasses three swirl inserts having single, double, and triple helicoid vanes of
Sw = 0.75 with dimensionless jet exit to impinging surface distance of 1, 2, 3, and 4. While the emphasis is
on understanding the flow attributes by varying the number of vanes, the circular impinging jet (CIJ) is
considered for comparative study.

2 Experimental approach

2.1 Impinging jet facility

The schematic of experimental facility is shown in Fig. 1. A high-pressure blower draws air and supplies to
a smooth pipe through a heat exchanger and venturi meter. The PID temperature controller with heat
exchanger maintains the uniform air temperature in the pipe. The regulating valve controls air flow rate and
venturi meter with differential manometer measures the discharge of air in the pipe. A smooth transition
pipe of 31 mm inner diameter and 830 mm length connecting the main pipe holds the swirl insert. The air
delivered from the smooth pipe impinges perpendicular to the impinging surface. The impinging surface
consists of thin electric heater sheet, stainless steel foil, and thermochromic liquid crystal (TLC) sheet.
A TLC sheet, electric heater sheet, and thin stainless steel foil composed together to form an impinging
surface. The stainless steel sheet has a size of 300 9 300 9 0.03 mm3 and the thermochromic liquid sheet
has a width and length of 300 mm each has a thickness of 0.1 mm. An acrylic plate of 450 mm square
section (thickness 12 mm) holds an asbestos plate and impinging surface by clamping screws. The average
temperature of experimental environment is 304 K. The air temperature inside the pipe is set to 308 K as the
working temperature of thermochromic liquid crystal is between 308 K and 322 K. The average temperature
of impinging surface is 313.5 K. A calibrated DC power supply unit (Model No: PSD 3203—Scientific Mes
Technik Limited) having current and voltage range of 0–3 A and 0–30 V, respectively, is employed for
heating the impinging surface. A canon digital camera (Model: EOS 600D) with spatial resolution of 2.83
pixels/mm and pixel array of 5184 9 3456 is used to obtain the images of color pattern in the TLC sheet.

2.2 Calibration

The calibration arrangement consists of acrylic plate of size 400 mm 9 400 mm 9 12 mm, aluminium
plate of size 300 mm 9 300 mm 9 3 mm, heater sheet, and TLC sheet to be calibrated. The surface of the
aluminium plate is provided with eight grooves to enable for placing thermocouples. Calibrated

Impinging
Differential Power
surface
U tube supply
+ unit
manometer Helicoid insert

Venturi meter y
Pt -100 x
Smooth
transition pipe temperature
sensor

Acrylic
Heat Temperature sheet
(PID)
exchanger
controller
High pressure
Blower
Flow control
valve

Fig. 1 Schematic of experimental setup

 S. Mohamed Illyas et al.

thermocouples (K-type) are placed in the grooves using a thermally conductive adhesive. NI 9213 data
logger is connected with thermocouples and DC power supply unit is connected with terminals of heater
sheet. The calibration of TLC sheet is carried out by simultaneously recording its surface temperature and
the color pattern while heating it in small incremental steps (308–321 K). The recorded color pattern in
RGB domain is converted to HSI domain using vision assistant module in image processing tool. While hue
represents pure color (Gonzalez et al. (2013)), TLC temperature versus normalized hue is plotted and
correlation relating temperature with hue value is obtained. The correlation is used to convert the hue
distribution of TLC into its surface temperature.

2.3 Helicoid insert

The swirl device used in the present work has helicoid surfaces that wrap around the cylindrical sec-
tion. Helicoid represents a three-dimensional helical surface rotating about z axis. Three kinds of helicoid
inserts have been used in this study. The first one consists of single-guide vane with one helicoid ribbon
attached to the cylindrical surface; the second kind of insert has two guide vanes spaced 180 apart, whereas
the third one consists of three guide vanes spaced 120 apart. A sharp cone is provided at the top of the
cylindrical part intended to minimize the drag Bakirci and Bilen (2007), as shown in Fig. 2. The helicoid
inserts (Table 1) are produced by rapid pro typing machine (Object Eden 350 V) using a polymer with
surface roughness of 5 lm. Swirl number is the measure of degree of swirl generated by helicoid surfaces,
which is calculated from the Eq. (1) given by the following Gupta et al. (1984):

"  3 #
2 1  Dd
Sw ¼   tanh; ð1Þ
3 1 d 2
D

where D is the outer diameter of vane, d is the vane hub diameter, and h is helicoid vane angle.

2.4 Data reduction

Hallcrest TLC sheet with bandwidth of R35C1 W is used in this study. The bandwidth defines the tem-
perature range of TLC between start of red (35 C) and start of blue (36 C). However, the bandwidth is the
temperature range between start of red (35 C) and clearing point temperature (49 C). The color pattern on
the liquid crystal sheet recorded by a digital camera in RGB domain is converted to HSI domain as reported
in Sect. 2.2. The temperature of liquid crystal sheet is computed by via correlation relating temperature and
normalized hue obtained through calibration of TLC. The heat transfer coefficient and its corresponding
Nusselt number are determined using the Eqs. (2) and (3), respectively, considering the energy balance on
the target surface:
qinput  qconduction  qnatural convection  qradiation
h¼ ; ð2Þ
ðTs  Tj Þ
hD
Nu ¼ ; ð3Þ
ka

D
θ

d
(a) Single helicoid

(b) Double helicoid

(c) Triple helicoid

Fig. 2 Helicoid inserts of Sw = 0.75

Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

where qinput is heat dissipated on the impinging surface per unit area A, qconduction is heat conducted into
stainless steel and TLC sheet per unit area, qnaturalconvection is the natural convection loss per unit area, and
qradiation is radiation heat loss per unit area. Ts and Tj are the local surface temperature and jet exit
temperature, respectively. The predetermined centre is considered as the intersection of jet axis and
impinging surface. D is inner diameter of smooth transition pipe and ka is thermal conductivity of air. A is
area of stainless foil.

2.5 Uncertainty analysis

The uncertainty analysis is carried out as the method suggested by Kline and Mc Clintock (Holman (2007))
using the equation as follows:
"      #12
oR 2 2 oR 2 2 oR 2 2
DR ¼ ðDe1 Þ þ ðDe2 Þ þ. . . ðDen Þ ; ð4Þ
om1 om2 omn

where R is the derived quantity, m is the measured quantity, and De is the error level in the measured
magnitude. The uncertainty in convective heat transfer coefficient (h) can be obtained by substituting the
derived quantity (R) and measured quantity (m) in Eq. (4):
20  12 0   12 0   12 0   12 312
Q Q Q Q
6Bo AðTs Tj Þ C o
B AðTs Tj Þ C
o
B AðTs Tj Þ C
o
B AðTs Tj Þ C 7
6 C ðe4 Þ2 7
Dh ¼ 6B C ðe1 Þ2 þB C ðe2 Þ2 þB C ðe3 Þ2 þB 7:
4@ oQ A @ oA A @ oTs A @ oTj A 5

ð5Þ
The maximum uncertainty in the Nusselt number is evaluated as 4.2% for single helicoid for
Re = 12,700 at H/D = 4. The uncertainty in Nusselt number includes the effect of error level in the
measurement of impinging surface temperature which is calculated by the method suggested by Geers et al.
(2008).

2.6 Local heat transfer and flow visualization

The local heat transfer rate and flow patterns of circular and swirling jets (Sw = 0.75) on the impinging
surface are presented in Figs. 3 and 4. The color of TLC (Fig. 3) shows the variation of Nusselt number
evaluated through its local surface temperature corresponding to hue value of the pixel on the impinging
surface. The oil film technique is used to obtain the flow pattern on the impinging surface. The liquid
mixture is prepared by mixing oleic acid with liquid paraffin and titanium dioxide. An acrylic sheet is used
as impinging surface and it is uniformly painted with oil film. The oil film image is obtained in RGB mode,
as that employed in obtaining image of TLC. An unremoved oil film is represented by white area, while
black area is the oil film removed region by the impinging jet. The region corresponds to oil film removed
from the wall is due intense shear flow by the impinging jet. The concentrated Nusselt number in the centre
of impinging region for CIJ implies that the presence of potential core maintains the velocity at exit and the
very little indication of jet spreading before impingement, as shown in Fig. 3a, which can also be visualized
by concentrated oil film removed region at the centre of the impingement (Fig. 4a). The concentrated heat
transfer in the region corresponds to X/D = 0–1.5 and Y/D = 0–1.5 at Re = 12,700 (Fig. 3b) which shows
the deviation of peak Nusselt number from the axis of jet for the single helicoid which can be better
visualized by oil film removed region resulting in non-uniform cooling over the impinging surface (Fig. 4b).
The intensity increases with increase in Reynolds number. In this way, the location of peak Nusselt number
moves away radially from the stagnation point because of tangential velocity component of the swirling jet.
Figure 3c, d shows the Nusselt number contour revealing the profile of the jet for double and triple helicoid
inserts which has higher heat transfer in the vicinity of stagnation region, respectively. The presence of
multichannels in the insert splits the flow into separate jets. The exit flow separated from inner circular rod
(hub) rolls up to form a recirculating zone at the centre of impinging area causing separation of jets (Fig. 4c,
d). The intensity of swirl generated is directly proportional to the helicoid vane angle (Gupta et al. (1984)).
Though the helicoid angle has very little effect on angle of separating the flow, it has a significant effect on
 S. Mohamed Illyas et al.

forming an intense axial pressure gradient causing axial recirculation zone at the centre of the impinging
surface (Fig. 4c, d) as shown by Ianiro and Cardone (2012). The further study by the authors (Mohamed
Illyas et al.) with swirl insert of Sw = 1.1 substantiates the presence of stronger axial recirculation zones in
the flow field. The axial recirculation zones are apparent for double and triple helicoids as seen with an
unremoved oil film at the centre of the impinging surface, as shown in Fig. 4c, d, respectively. Figure 3
shows the regions of peak heat transfer caused by the swirling jet becomes decentralized and stronger due to
swirl momentum and jet dispersion with higher degree. The magnitude of Nusselt number on impinged area
increases with increase in Reynolds number, as shown in Fig. 3.

2.7 Numerical methodology

Numerical simulation is carried out using commercial tool Ansys CFX. Governing equations of steady
incompressible flow are solved to obtain the solution. The equations are discretized by applying finite
control volume method. The computational domain is discretized with tetrahedral and prism ele-
ments (Fig. 6). To resolve the high-velocity gradients, a fine grid is used at the flow exit and near
impingement region. The mesh intensity becomes coarser with increasing radial distance. The advection

Re = 12700 Re = 17900 Re = 23100 Re = 28300 Re = 32700

(a) Circular Impinging Jet

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(b) Single helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(c) Double helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(d) Triple helicoid

Fig. 3 Local Nusselt number distribution on the impinged surface at H/D = 2

Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(a) Circular Impinging Jet

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(b) Single helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(c) Double helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(d) Triple helicoid

Fig. 4 Flow visualization by oil film technique at H/D = 2

term is discretized with second-order high resolution. Though the second-order discretization scheme gives
more precise results, it is relatively less stable compared to the first order with respect to convergence. To
enhance the convergence, the solution for the whole domain is initially obtained with first-order (blend
factor = 0) upwind discretization method with the residual value of 10-6 and the blend factor is consec-
utively increased up to 1 with initialized values of earlier results to ensure second-order high-resolution
scheme at the final step of the solution with the similar convergence criteria. The details of boundary
conditions, selection of turbulence model, and grid independency test are subsequently discussed. Figure 5
presents the computational domain and its boundary conditions used in this simulation. The diameter of the
jet pipe is D. The ratio of the distance of impinging surface from jet exit and the diameter of jet pipe, H/D, is
equal to 2. To study the influence of separation distance on flow fields, H/D up to 4 is considered.
The fluid employed is air at the temperature of 308 K and supplied to the impinging surface with a static
pressure of 1 atm. The inlet boundary condition is set as a constant velocity inlet corresponding to the
 S. Mohamed Illyas et al.

Pressure
outlet

Confined
adiabatic wall

y
x
constant heat flux
Velocity inlet D
wall

Confined
adiabatic wall

Pressure
outlet

Fig. 5 Computational domain and boundary conditions

Table 1 Parameters of helicoid insert and pipe

Parameter Value
Diameter of helicoid insert (outer diameter of vane), D 29.5 mm
Diameter of hub, d 7 mm
Length of insert, L 198 mm
Helicoid vane angle, h 47
Clearance between helicoid insert and pipe inner diameter 1.5 mm

Reynolds number of 23,000 based on the average velocity of jet. The pressure outlet condition is applied
with an average pressure of 1 atm. The impinging surface is assigned with constant heat flux wall boundary
condition. The confined walls are set with no slip adiabatic conditions.
Figure 7 shows turbulence model evaluated in the present study which compares the axial velocity
variation with the corresponding experimental data obtained by Fenot et al. (2015). While standard SSG Re
stress, k-e and RNG k-e models fail to predict the experimental data in the stagnation region and standard
k-x and SST k-x models fairly predict the experimental results with a relative error of 4.03 and 3.97%,
respectively, corresponding to the region 0 \ r/D \ 0.6. While the assumption of isotropic turbulence in k-e
model and the use of wall functions poorly approximate the near wall velocity fluctuations and associated

Fig. 6 a, b Mesh distribution of helicoid insert and jet impingement channel

Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Fig. 7 Axial velocity variation of CIJ for different turbulence models at H/D = 0.02

Table 2 Range of parameters and turbulence models in selected numerical studies

Literature Re H/D Turbulence model

Ahmed et al. (2015) 23,000 0.5–1.9 RNG k-e
Alimohammadi et al. (2014) 6000–14,000 1–6 SST with transition model
Caggese et al. (2013) 16,500–41,800 0.5–1.5 SST
Ortega Casanova (2012) 7000–20,000 5,10 and 30 SST k-x
Ramezanpour et al. (2007) 4000–16,000 4-10 RNG k-e

transport properties, whereas the RNG k-e model incorporates an additional term in the turbulent kinetic
dissipation based on strain rates offers slightly better performance; and Heck et al. (2001) showed that RNG
k-e model gives comparable results of Nu in the wall jet region but an error up to 10% in the stagnation
region. The SST k-x model works well with two equations of k-x and k-e, where the former is used for flow
computations near the wall where the viscous sublayer prevails switches to the latter for fully turbulent flow
field with a blending function ensuring smooth transition between these two models (Ansys 2011).
Table 2 compares the turbulence model employed in the previous studies. The RNG k-e model and
standard k-x models perform moderately well compared with standard k-e model in predicting the flows
with strong pressure gradients and streamline curvature subjected to limitation of isotropic eddy viscosity
assumption. The SST with transition model is reliable for transition flows (Alimohammadi et al. 2014).
While SST k-x model accounts for the transport of the turbulent shear stress and gives highly accurate
predictions of the onset and the amount of flow separation under adverse pressure gradients (Ansys (2011))
for flows involving rotation and recirculation (Liu et al. (2014)), it is preferred in the present study.
A comprehensive study on the grid density is carried out for validating the grid independence of the
numerical result. This is performed by successively fining the grids from a baseline mesh of 1.19 millions to
fine mesh of 6.13 millions. The case with circular impinging jet with H/D = 2 for Re = 23,000 is considered
by employing SST k-x turbulence model. Figure 8 shows the variation in axial velocity for the five different
meshes. The deviation is observed in the region corresponds to 0.4 \ r/D \ 0.9 and Table 3 presents the
relative deviation of axial velocity with varying grid and it is minimal for the grid factor F4 from the
reference grid factor F5 which shows that increasing the grid nodes larger than F4 will have marginal effect
on the extent of deviation. Thus, grid nodes corresponding to F4 is employed in the present study to balance
the accuracy and computational time, and it remains constant as long as the size of domain is not changed.
The presence of wider potential core in the jet close to nozzle exit (H/D = 0.02) causing uniform centre
line velocity (Viskanta 1993) and loses its axial momentum when the jet spreads resulting in decay of axial
velocity as presented in Fig. 7. Whereas the axial velocity is initially increasing at H/D = 2, as shown in
Fig. 8. This is due to the fact that when the flow goes upstream of nozzle exit, the imbalance between the
centrifugal force and the radial pressure gradient sets up secondary motion. The fluid near the pipe axis
moves outward, while the fluid near the top and bottom walls moves inward. This results in the distortion of
axial momentum distribution with higher velocity occurring downstream of the pipe exit (Ferdman et al.
2000). This phenomenon is possible when the jet at exit has sufficient room to develop such as larger
separation distances.
 S. Mohamed Illyas et al.

Fig. 8 Axial velocity variation of CIJ for varying grid intensities for Re = 23,000 at H/D = 2

Table 3 Data showing different grids and their axial velocity variation from grid factor F5

Grid factor Grid nodes million Avg. u/Uo Deviation of Avg. u/Uo %
F1 1.19 0.242 10.73
F2 2.39 0.250 7.69
F3 3.61 0.259 4.31
F4 4.84 0.266 1.77
F5 6.13 0.271 –

In Fig. 9, the obtained values of axial velocity variation of circular impinging jet for Re = 23,000 at H/
D = 0.02 are compared with the corresponding experimental results reported by Fenot et al. (2015), Fair-
weather and Hargrave (2002), and Alekseenko and Bilsky (2007); as observed, the results fairly agree with
those in earlier published literatures.

2.8 Effect of number of helicoid vanes on axial velocity

Figure 10 compares the axial component of mean velocity near the impingement surface for circular
impinging jet (CIJ) and swirling jet at H/D distance of 1–4 for Sw = 0.75. The axial velocity distribution of
CIJ is higher in the region corresponds to 0 B r/D B 0.6 and decreases sharply beyond that region, as shown
in Fig. 10, causes reduced heat transfer rate resulting in non-uniform cooling over the impinging surface.
Though the axial velocity produced by the swirling jet is relatively lower which extends radially in the
impinging region corresponds to 0 B r/D B 1.7 indicating uniform jet spread which may have positive
impact on heat transfer distribution producing fairly uniform cooling over the surface. The axial velocity
component of swirling jet significantly differs from CIJ as it reaches a negative value in the region

Fig. 9 Comparison of axial velocity variation with literature data

Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Fig. 10 a, b, c, d Axial velocity distribution at H/D = 1, 2, 3, and 4 for circular and swirling jets of Sw = 0.75 for Re = 23,000

corresponding to 0.33 B r/D B 0.5 at H/D = 1 and it increases in the radial direction which reaches its
maximum value; thereafter, it decays in the wall jet region.
The negative value of the axial velocity is due to vortex break down causing recirculation zones as
assessed by Alekseenko and Bilsky (2007) Fenot et al. (2015) and the increase in its axial velocity is due to
the imparting of kinetic energy to the fluid by the helicoid vanes. While increasing the H/D distance, the
magnitude of axial velocity for CIJ is still higher than swirling jet in the stagnation region substantiating the
existence of potential core where as the swirling jet loses its axial momentum resulting in flatter axial
velocity profile as assessed by Mahmood (1980) with a magnitude near zero in the entire region. The higher
spreading rate of swirling jet results in earlier breakdown of potential core which lowers its axial velocity in
the stagnation region at H/D = 1, as shown in Fig. 10a, agreeing with the observation of Mahmood (1980)
and Chigier and Chervinsky (1967) on the non-existence of potential core even at weak swirl (Sw = 0.12
and 0.134) at H/D = 2. When the jet moves radially outwards, the axial velocity component becomes
inferior to zero and increases gradually to reach its maximum value; thereafter, it reduces to near zero.
The increase in its axial velocity is due to the fact that the helicoid vanes impart kinetic energy to the jet.
The strong recirculation zone of jet leaving the triple helicoid causes the relatively lower axial velocity
component in the impinging region compared with single and double helicoids. When increasing the
separation distance (H/D = 2), the axial velocity of swirling jet reaches below zero (Fig. 10b) in the region
corresponds to 0 B r/D B 0.7 which shows the presence of recirculation zone. The recirculation zone is
noticeably larger when compared with H/D = 1. The rotation of swirling jet imparts centrifugal effect on the
flow resulting in radially increased recirculation zone as reported by Fenot et al. (2015). The axial velocity
of the jet leaving the triple helicoid is higher in the region 0.77 B r/D B 1.3 compared with single and
double helicoids. This is attributed to the fact that higher kinetic energy imparted to the fluid by the triple
helicoid vanes as reported earlier. The swirling jet leaving the helicoid vanes substantiates the existence of
vortex break down in the impinging region resulting in negative value of axial velocity at H/D = 3, as shown
in Fig. 10c. The increase in axial velocity for the swirling jet is less intense when compared to H/D = 2
causing peak axial velocity moving away radially at r/D = 1.77, 1.57, and 1.43, respectively, for single,
double, and triple helicoids.
While increasing the H/D distance further (H/D = 4), the axial velocity profile becomes flatter with its
magnitude nearing zero for the jet leaving single and double helicoids, as shown in Fig. 10d. This is
 S. Mohamed Illyas et al.

attributed to the lower axial momentum of the jet at the increased separation distance. With stronger swirl,
the axial pressure gradient of jet leaving triple helicoid is strong enough to produce recirculation zone
resulting in negative axial velocity in the region 0 B r/D B 0.77 for the jet leaving triple helicoid, whereas
the vortex breakdown effect is probably not seen for single and double helicoids resulting in positive axial
velocity.

2.9 Effect of number of helicoid vanes on tangential velocity

Figure 11 presents the tangential component of velocity for the CIJ, single, double, and triple helicoids of
Sw = 0.75 at H/D = 1, 2, 3, 4. The results show that the tangential velocity is near zero for CIJ at H/D = 1,
whereas the tangential velocity is significantly higher for the swirling jet in the impinging region and decays
with increasing r/D from the stagnation region. The increase in tangential velocity is due to strong swirl
momentum exerted by the helicoid vanes. While increasing the H/D distances, no significant change in
velocity profile is observed for CIJ and the increase in tangential velocity is less intense for the swirling jet;
and it is minimum at H/D = 4. A significant difference in the tangential velocity profile among the swirling
jets is observed in the region 0 B r/D B 0.9 at H/D = 1, as shown in Fig. 11a. The higher spreading rate of
jet leaving double and triple helicoids causes steep increase in tangential velocity in the region 0 B r/
D B 0.5, whereas the rise is less prominent for single helicoid relatively with lower spreading rate. The
result reveals a higher tangential velocity for the swirling jet within the recirculation region which agrees
with findings of Karuppa Raj and Ganesan (2009). The larger intensity of tangential velocity for triple
helicoid means the jet tending to spread out radially more than the rest of the helicoids (single and double
helicoids). The average tangential velocity of the jet leaving single, double, and triple helicoids reduces by
24.2, 14.7, and 22.9%, respectively, compared with H/D = 1. The decrease in tangential component of
velocity is due to reduced swirl momentum of jet. A significant reduction in tangential velocity for swirling
jet is observed at H/D = 3. Besides, the velocity peak is 1–1.2 times smaller than with those obtained for H/
D = 2. It appears that, with relatively strong swirl momentum, the triple helicoid exhibits a larger intensity
of velocity compared with single and double helicoids. While increasing the H/D distances further (H/
D = 4), the jets leaving the single and double helicoids with lower swirl momentum exhibit a flat velocity
profile in the entire region and the intensity is marginally higher for triple helicoid.

Fig. 11 a, b, c, d Tangential velocity distribution at H/D = 1, 2, 3, and 4 for circular and swirling jets of Sw = 0.75 for
Re = 23,000
Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

2.10 Effect of number of helicoid vanes on turbulent kinetic energy

Figure 12 shows the mean turbulent kinetic energy distribution near the impinging surface for CIJ and
swirling jet at H/D distances of 1–4 for Sw = 0.75. The intensity of turbulence for the swirling jet
approaching the impinging surface is higher than circular jet.
The momentous increase in turbulence indicates that a higher rate mixing for the jet under forced
condition. The effect of swirl exerted by helicoid vanes extends the flow radially outward through the
ambient air causing increasing in turbulence level radially in the region corresponding 0 \ r/D \ 1.5 at H/
D = 1. The swirling jet losing its tangential momentum results in reduction in its intensity beyond the region
r/D = 1.5, as shown in Fig. 12a. It is evident from Fig. 12a that the spreading of swirling jet is minimum at
H/D = 1, whereas the swirling effect is more evident with sufficient room for the jet to spread causing
relatively higher turbulence level at H/D = 2, as shown in Fig. 12b. The location of the peak also moves
radially outward at r/D & 1.1–1.3 indicating uniform spreading of jet. It is observed from Fig. 12b that the
intensity of turbulence relatively higher in the region r/D = 0–1.4 for the triple helicoid compared to the
single and double helicoids. This is attributed to strong swirl momentum exerted by the multiple helicoid
vanes. The presence of number of helicoid vanes significantly affect the change in kinetic energy distri-
bution at H/D = 3. The swirling jet leaving the triple helicoid maintaining its original momentum causes
relatively higher intensity of turbulence in the impinging region, whereas the turbulence level reduces for
the single and double helicoids at H/D = 3 compared with H/D = 2. The location of peak further moves
away radially at r/D & 1.7, 1.6, and 1.4 for single, double, and triple helicoids, respectively, as shown in
Fig. 12c. While increasing the separation distance further (H/D = 4), the swirling jet loses its tangential
momentum exhibiting lower turbulence kinetic energy distribution over the impinging surface radially.
While the reduced swirl momentum exerted by the double helicoid resembles the turbulence distribution of
its circular jet counterpart, the mixing jet leaving the single helicoid with ambient air results in monotonic
decrease of turbulence level, whereas the distribution of turbulence is relatively higher for triple helicoid
maintaining its momentum for H/D = 4, as shown in Fig. 12d

Fig. 12 a, b, c, d Mean turbulent kinetic energy distribution at H/D = 1, 2, 3, and 4 for circular and swirling jets of Sw = 0.75
for Re = 23,000
 S. Mohamed Illyas et al.

2.11 Turbulence intensity

Figure 13 presents the radial component of turbulent intensity obtained with root mean square of radial
velocity fluctuation and mean velocity. The inclusion of swirl into the impinging jet significantly affects the
turbulence behavior from its circular jet counterpart. The increased jet spread for the swirling flow influ-
ences the magnitude and location of turbulence in the wall jet region, as shown in Fig. 13. The higher radial
component of turbulence intensity due to the strong swirl momentum of swirling jet enhances the turbulence
radially in the region corresponds to 1 B r/D B 4.5, as shown in Fig. 13, which may have positive impact
on the heat transfer. The higher turbulence intensity near the stagnation region for the circular jet responsible
for local maximum Nusselt number and its magnitude decreases radially, as shown in Fig. 13a–d, which is
in agreement with results of Nusselt number distribution presented experimentally and numerically in
Figs. 3a and 14a, respectively. The aspect of jet spread and intense flow mixing in the case of swirling jet at
the downstream of nozzle exit enhances the turbulence which extends in the wall jet region resulting in
higher magnitude of radial component of turbulence intensity, as shown in Fig. 13, and probably contributes
to increased radial uniformity of heat transfer on the impinging surface which is comparable with the results
of Nusselt number distribution presented in Figs. 3b–d and 14b–d. The turbulent intensity in the radial
direction is larger for the swirling jet particularly close to the jet axis where the shear layer exists at H/D
distances of 1 and 2. This fact may attribute to the formation of recirculation flows associating with other
components of turbulent intensities as shown by Mamoru Senda et al. (2005) in their study for Sw = 0.22
and 0.45, while the higher intensity of turbulence for the swirling jet is observed near the impinging surface
at lower H/D distances (1 and 2) with its maxima located in the region which corresponds 1 B r/D B 1.5
and its magnitude decreases with increased H/D distances widening radially in the region 2 B r/D B 4.5, as
shown in Fig. 13. Whereas, with the higher axial momentum exerted by the circular jet, the reduction in
intensity of turbulence is marginal near the impinging surface at the increased H/D distances, as shown in
Fig. 13a–d. The jet leaving the single and double helicoids with lower swirl momentum mixing with
ambient air resembles the turbulence distribution of circular jet near the impinging surface, whereas, for the
triple helicoid with higher swirl momentum, the turbulence level is relatively higher at H/D = 4 (Fig. 13p)
as reported earlier in Sect. 3.3.

2.12 Local Nusselt number distribution

The local Nusselt number distribution obtained through numerical simulation is presented in Fig. 14. The
concentrated Nusselt number at the centre of impinging region for CIJ reveals that its intensity increases
with increase in Reynolds number, as shown in Fig. 13a, which is in agreement with the Nusselt number
distribution experimentally obtained in Fig. 3a. The swirling jet broadens the impinging region, thereby
enhancing the heat transfer rate as observed in Fig. 14b–d. The broadening of impinging region is attributed
to tangential momentum exerted by the helicoid vanes as reported earlier. The concentrated heat transfer
region for the single helicoid radially moves away from the jet axis due to swirling effect exerted by single
helicoid vane, as shown in Fig. 14b. In the case double and triple helicoids, the peak heat transfer regions
are separated at the jet axis due to blockage created by insert hub, as shown in Fig. 14c, d, which are
comparable with the Nusselt number distribution obtained experimentally presented in Fig. 3c, d.

2.13 Effect of number of helicoid vanes on vorticity distribution

Figure 15 shows the average vorticity distribution for CIJ and swirling jet for H/D distances of 1, 2, 3, and 4.
The presence of strong vorticity is apparent for CIJ at the boundary of jet which is due to velocity gradients
in the jet causing shear layer at the edges of the jet. The circular jet leaving the pipe with velocity gradient
creates shearing at the edges of jet resulting in development of vortices at the boundary of jet, as shown in
Fig. 15a, at H/D = 1. The lower turbulence of circular jet leaving the pipe remains unaffected the potential
core region as no vortices are observed in the free jet region. The vorticity distribution for the swirling jet
shows the presence of vortices at the edges of the swirling jet as well as at the free jet region at H/D = 1
(Fig. 15e, i, m). The presence of vortices at the free jet region is due to fact that the higher turbulence of
swirling jet results in breakdown of the potential core. While increasing the H/D distance further, the jet
loses its momentum with lower magnitude of shear layer resulting in weak vortices at the jet edges, as
shown in Fig. 15b–d, for CIJ. No vortices are observed for CIJ in the region r/D = 0–0.7 and it increases
rapidly beyond that region at lower H/D distances (H/D = 1 and 2), whereas, at the increased H/D distances,
Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Fig. 13 Radial intensity of turbulence (Ir = ur0 /Uo) for circular and swirling jets at H/D = 1, 2, 3, and 4 for Re = 23,000
 S. Mohamed Illyas et al.

Re = 12700 Re = 17900 Re = 23100 Re = 28300 Re = 32700

(a) Circular Impinging Jet

Re = 12700 Re = 17900
R Re = 28300 Re = 32700
32 00
Re = 23100
(b) Single helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(c) Double helicoid

Re = 12700 Re = 17900 Re = 28300 Re = 32700

Re = 23100
(d) Triple helicoid

Fig. 14 Local Nusselt number distribution on the impinged surface at H/D = 2

shear layer develops inward to the centre of the jet resulting in marginal increase in the magnitude of
vorticity in the (stagnation region) region r/D = 0.25. On the contrary, the vorticity distribution is radially
uniform for the swirling jet at lower H/D distances (H/D = 1 and 2) and its magnitude reduces at increased
H/D distances. The coherent rotating flow structures are apparent for the jet leaving single, double, and triple
helicoids along its flow path which is entirely due to the existence of velocity gradient that establishes shear
layer in the free jet region at H/D = 1, as shown in Fig. 15e, i, m. The increased turbulence when the flow
turns further downstream in the wall jet region establishes vorticity in the region corresponds to r/
D = 0.5–2. While increasing the H/D distance further (H/D = 2), the vorticity in the flow structure is more
intense till at H/D = 1.25 at downstream for the single helicoid, as shown in Fig. 15f, whereas it is more
pronounced further downstream till at H/D = 1.5 for double and triple helicoids (Fig. 15j, n) which can be
responsible for entraining of more ambient air and spreading of jet as reported by O’Donovan (2005). The
jet leaving the double helicoid exhibits more symmetric flow structure which may possibly enhance uni-
formity in flow distribution on the impinging surface. The flow leaving the single helicoid loses its
momentum causing a lower intensity of vorticity distribution at downstream of flow beyond H/D & 2 for
Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Fig. 15 Flow field showing distribution of average vorticity (x’D/Uo) for circular and swirling jets at H/D = 1, 2, 3, and 4 for
Re = 23,000

single helicoid at H/D = 3 (Fig. 15g), whereas, with strong swirl momentum ,it is relatively higher beyond
H/D & 2.25 and 2.5 for double and triple helicoids, respectively (Fig. 15k, o). The symmetric flow structure
of jet leaving the double helicoid (Fig. 15k) is still apparent with lower magnitude of vorticity compared
with H/D = 2. With further increase in separation distance (H/D = 4), the jets leaving the single and double
helicoids lose their original momentum causing mixing of jet resulting in lower intensity of vorticity
distribution beyond H/D = 3, as shown in Fig. 15h, l, whereas the swirling effect of jet leaving triple
helicoid (Fig. 15p) is strong enough to maintain its momentum resulting in relatively higher vorticity
distribution near the impinging surface.

2.14 Effect of number of helicoid vanes on velocity vectors

Figure 16 presents the average velocity vectors for CIJ and swirling jet for H/D distances 1, 2, 3, and 4.
Figure 16 a illustrates that the presence of potential core is evident as the flow is not agitated in the free jet
region. As the jet moves downstream, the flow turns radially at the impinging surface and the cross flow is
 S. Mohamed Illyas et al.

Fig. 16 Flow field showing average velocity vectors for circular and swirling jets at H/D = 1, 2, 3, and 4 for Re = 23,000

formed at H/D = 0.5. The cross flow effect is due to the interaction of wall jet flow with the surrounding
fluid. When the separation distance is increased, the effect of cross flow reduces and the recirculation zones
are observed; and it becomes larger at H/D = 4 (Fig. 16d). Whereas the swirling effect of jet leaving the
helicoid inserts causes stronger axial pressure gradient as assessed by Ianiro and Cardone (2012) resulting in
axial recirculation zones at the flow downstream and hence the potential core no longer exists as shown in
Fig. 16. The cross flow effect is evident at lower H/D distances as encountered in CIJ. The swirling jet
exhibits weaker recirculation zones at the increased H/D distances probably with lower swirl momentum.
The jets leaving the single, double, and triple helicoids substantiate the existence of two shear layers in
the jet at H/D = 1, as shown in Fig. 16e, i, m: an outer layer in the region between the jets and the ambient
Heat transfer and flow visualization of swirling impinging jet on flat surface using helicoid inserts

Table 4 Performance parameters

Description Non-uniformity index Mean turbulent kinetic energy (m2/s2) Pressure drop (N/m2)
Circular jet 0.18 0.564 490.00
Swirling jet—single helicoid 0.06 1.18 597.69
Swirling jet—double helicoid 0.07 1.64 676.08
Swirling jet—triple helicoid 0.11 1.73 774.06

fluid and an inner layer in the region between the jets and axial recirculation zones which agrees with the
findings of Fenot et al. (2015) for swirling jet for Re = 23,000 at H/D = 1 and Ianiro and Cardone (2012) or
Re = 28,000 at H/D = 2. The existence of inner and outer shear layers is more intense for the swirling jet
increasing its size axially and radially at H/D = 2, as shown in Fig. 16f, j, n. The reverse upstream is more
coherent in the inner region of the flow for single and double helicoids, whereas the multiple jets leaving the
triple helicoid form more recirculation zones causing incongruent flow structure in the inner region resulting
in increased turbulence. The increased turbulence of jet may cause more entrainment of ambient air. The
effect of cross flow reduces at H/D = 3 and secondary vortices roll up near the side wall (in front of the wall
jet region) for the swirling jet, as shown in Fig. 16g, k, o. The recirculation zone becomes larger for the
swirling jet with the presence of inner and outer shear layers in the flow field. The symmetric reverse
upstream is evident in the inner region of the jets leaving single and double helicoids (Fig. 16g, k). The
vortices roll up in the inner region of the swirling jet and it is more intense for the triple helicoid resulting in
more turbulence as shown in Fig. 16o. The size of secondary vortices becomes larger for the swirling jet
(Fig. 16h, l, p) at H/D = 4 and marginally affects the outer shear layer of impinging jet; and it is more
intense for single helicoid.

2.15 Quantitative comparison of performance parameters

While the presence of helicoid vane causes pressure drop in the flow, an effort has been taken to ascertain
whether the justification can be made on compensating the pressure drop by enhancing heat transfer
performance and promoting turbulence. Table 4 presents the quantitative measure of performance param-
eters for circular and swirling impinging jets for Re = 23,000 at H/D = 3. The experimentally obtained
pressure drop values for the helicoid insert are consistent with those found in the numerical study by
Salvador B. Rodriguez (2011) on elimination of hotspots in very high temperature reactor using quadruple
helicoid insert. To characterize the heat transfer performance of impinging jet, a non-uniformity index is
computed with the experimentally obtained Nusselt  number. The non-uniformity index is obtained by
calculating normalized standard deviation r ¼ r ¼ Nuravg as given by Wang et al. (2005), where r is
standard deviation of Nusselt number over the whole surface. The numerically obtained mean turbulent
kinetic energy is presented to quantify the turbulence effect. The non-uniformity index and turbulence level
of swirling jet are comparable with circular jet.
The quantitative comparison of parameters presented in Table 4 comprehend that the decrease in non-
uniformity index for the swirling jet is 125% and increase in mean turbulent kinetic energy is 170% with
those obtained with circular jet; and the increase in pressure drop for the swirling jet is 39.3%. The pressure
drop may be justified substantiating the benefits gained related to heat transfer and turbulence as a result of
helicoid insert.

3 Conclusion

The heat transfer and flow characteristics of swirling impinging jets on a flat surface are studied and the
following conclusions have been drawn.
• The presence of potential core at the downstream of circular impinging jet causes higher axial
component of mean velocity in the stagnation region and rapid decay in the wall jet region due to
increased velocity gradient which may adversely affect the heat transfer characteristics.
• At lower H/D distances (H/D = 1 and 2), the axial velocity of swirling jet reaches negative value in the
stagnation region and increases rapidly in the region at r/D = 0.4–1 and exhibiting flat axial velocity
profile at increased H/D distance of 4. The magnitude of negative axial velocity component is relatively
higher for the triple helicoid at H/D = 4 distance due to the presence of strong axial recirculation zones.
 S. Mohamed Illyas et al.

• The tangential velocity component is near zero for circular impinging jet, whereas with higher swirling
effect, the triple helicoid exhibits higher tangential velocity in the region at r/D = 0–0.4, 0–0.8, and
0–1.4 at H/D = 1, 2, and 3, respectively, compared with single and double helicoids and the swirling jet
loses its momentum exhibiting flat tangential velocity profile at H/D = 4.
• No vorticity distribution is observed in the inner part of free jet region and stagnation region of CIJ,
whereas it is more coherent and pronounced for the swirling jet at lower H/D distances (H/D = 1 and 2).
The presence of vorticity distribution is more apparent for triple helicoid at downstream of the jet near
the wall jet region causing to entrain more ambient air, whereas it is less intense for the single and
double helicoids at H/D = 4.
• The higher swirl momentum exerted by triple helicoid vanes causes relatively increased turbulent kinetic
energy distribution on the impinging surface compared with single and double helicoid vanes.
• Higher intensity of turbulence is observed near the impinging surface for the swirling jet at lower H/D
distances (H/D = 1 and 2) in the region 1 B r/D B 1.5 and its magnitude reduces with increased H/D
distances widening radially in the region 2 B r/D B 4.5, whereas the intensity is relatively lower for the
circular jet exhibiting maximum near stagnation region and with lower intensity thereafter.
• The existence of inner and outer shear layers are evident for the swirling jet at H/D distances of 1, 2, and
3, whereas the inner shear layer no longer exists in the presence of potential core in the case of CIJ. The
development of large-scale vortices in front of the wall jet region is evident for single and double
helicoids at H/D = 4, whereas the vortices are relatively small for the triple helicoid, marginally
affecting the inner shear layer.
• The concentrated heat transfer in the region corresponds to X/D = 0–1.5 and Y/D = 0–1.5 at H/D = 2
which represents the deviation of peak Nusselt number from the axis of jet for the single helicoid
resulting in non-uniform cooling over the impinging surface.

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