CN108051475B - Rapid measurement method for convective heat transfer coefficient - Google Patents

Abstract

The invention discloses a rapid measurement method for a heat convection coefficient, which is suitable for measuring the heat convection coefficient of a medium surface under the condition of changing with temperature. The measurement method has the core idea that the measurement of the convective heat transfer coefficient is converted into the optimization problem of solving the unknown parameters of the boundary of the heat conduction problem, and the ultrasonic echo method is adopted according to the medium temperature-ultrasonic propagation characteristic, so that the surface flow heat transfer coefficient changing along with the temperature can be rapidly and nondestructively measured based on the solution of the inverse heat conduction problem; the method has the advantages of simple measuring device, short measuring period, avoidance of contact interference between the sensor and the tested piece, no limitation of the measuring range by the high-temperature resistance of the sensor and the like.

Description

Rapid measurement method for convective heat transfer coefficient

Technical Field

The invention relates to the field of ultrasonic detection, in particular to a rapid measurement method for a convective heat transfer coefficient.

Background

The convective heat transfer coefficient represents an important parameter of the convective strength of the fluid, and the accurate prediction thereof has important application value in the fields of building energy conservation, energy chemical engineering, casting processing and the like, but because the method is related to a plurality of factors such as the physical properties and the states of the fluid and the heat transfer surface, how to accurately obtain the convective heat transfer coefficient is one of the difficult problems in engineering.

The method for measuring the convective heat transfer coefficient mainly comprises a steady state method and a transient state method. The steady state method has simple principle and simple and convenient operation, but has long experimental period. The transient law has the advantages of short period and small error, but has the problems of complex experimental equipment and harsh experimental conditions (for example, the required solid temperature can be regarded as uniform). The method converts the measurement of the convective heat transfer coefficient into the optimization problem of solving the unknown boundary parameter of the heat conduction problem, and essentially belongs to a transient method. However, the method is based on an ultrasonic method, the required measuring device is simple, the operation is simple and convenient, and the advantages of short measuring period and high precision of a transient method are kept.

Disclosure of Invention

The invention aims to provide a method for rapidly measuring a convective heat transfer coefficient, which adopts an ultrasonic echo method to convert the measurement of the convective heat transfer coefficient into an optimization problem for solving unknown parameters of the boundary of a heat conduction problem according to the temperature-ultrasonic propagation characteristic of a medium, and can rapidly and nondestructively measure the surface flow heat transfer coefficient changing along with the temperature based on the solution of the inverse heat conduction problem.

In order to achieve the purpose, the invention adopts the following technical scheme:

the method comprises the following steps: and acquiring the relation between the propagation speed V of the ultrasonic wave in the tested piece and the temperature T based on a calibration experiment.

Step two: heating the test piece by ultrasonic pulse echo method_iTime of ultrasonic wave propagation time t_i,exp。

Step three: the measurement of the convective heat transfer coefficient is converted into an optimization problem for solving the unknown parameters of the boundary of the heat transfer problem. The following optimization objective function is used:

wherein: h is_cThe convective heat transfer coefficient is measured on the surface of the medium; t is t_i,calT calculated for a value_iThe ultrasonic wave propagation time of the moment, the measurement time sequence represented by subscript i, and N represents the total measurement time point number; l is the length of the test piece in the measured direction;

the constraint conditions are as follows:

wherein: k, C and rho are respectively the thermal conductivity, specific heat capacity and density of the material to be measured; t is_fThe temperature of the fluid medium is controlled by a heater; t is_tTemperature of the end for placing the ultrasonic probe, T₀The initial temperature of the test piece.

Step four: solving the inverse problem of heat conduction by adopting an adjoint equation algorithm to obtain the convective heat transfer coefficient of the medium surface.

The solution process of the optimization algorithm comprises the following steps:

(1) inputting thermophysical property parameters, dimensions and initial values of the dielectric material;

(2) solving the heat conduction equation to obtain the temperature field T (x, T) and the target function J (h)_c) A value of (d);

(3) the adjoint equation algorithm is adopted to optimize the parameter value to obtain better h_c；

(4) Judging whether convergence is achieved (the convergence is less than or equal to 1e-6), and stopping calculation if the convergence is achieved; otherwise, returning to the step (2) to repeat iteration until convergence is reached;

(5) obtaining the convective heat transfer coefficient h of the medium surface_c。

In summary, due to the adoption of the technical scheme, the invention has the beneficial effects that:

1. the ultrasonic equipment has the characteristics of simple measuring device, simple and convenient operation, repeated use and the like, and the method can obtain the convective heat transfer coefficients at different temperatures from room temperature to 500 ℃ only by measuring once, for example, heating the heating surface of a tested piece to a preset temperature value such as 500 ℃, so that the method has the advantages of high measuring speed, low cost and the like;

2. when non-contact measurement is carried out based on electromagnetism or laser ultrasound, the measurement of the convective heat transfer coefficient is hardly influenced by the temperature resistance of the sensor, and the method has the advantage of wide measurement range.

Drawings

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a flow chart of a convective heat transfer coefficient measurement method;

fig. 2 is a measurement of convective heat transfer coefficient as a function of temperature.

Detailed Description

All of the features disclosed in this specification, or all of the steps in any method or process so disclosed, may be combined in any combination, except combinations of features and/or steps that are mutually exclusive.

The ultrasonic probe is arranged on the upper end surface of a cylindrical test piece, and the pulse ultrasonic wave is excited in a vertical incidence mode based on the change of the echo propagation time in the test piece. The other surfaces are insulated except the lower end is contacted with the fluid, namely, the other surfaces are similar to one-dimensional problems. And inverting the convective heat transfer coefficient of the contact surface of the test piece and the fluid by solving the inverse problem of heat/sound coupling.

Case1 fluid temperature T-20 × (1+ T × 0.045) (° c), where T is time, convective heat transfer coefficient h of solid surface_cDoes not change with temperature, and has a true value of 165W/m²DEG C. The predicted result is consistent with the true value.

Case2 fluid temperature T200 × (1+ T × 0.0028.0028) (° c), where T is time, convective heat transfer coefficient h of solid surface_cThe temperature of room temperature to 200 ℃ is 165W/m²283W/m at the temperature of over 200 DEG C²DEG C. The predicted result is consistent with the true value.

Case3 fluid temperature T20 × (1+ T × 0.1.1) (° c), where T is time and the convective heat transfer coefficient of the solid surface is h_c＝0.0004×T²+0.4294×T+125.6(m²℃W^-1) Wherein T is temperature.

The material parameters are obtained by pre-fitting experimental data, and the convective heat transfer coefficient is usually not known a priori in the engineering practice, so that the convective heat transfer coefficient is expressed as a piecewise function which changes along with the position and the time in a heat transfer model,

and the function is given through parameter identification, and the specific calculation flow is shown in fig. 1.

Fig. 2 shows the convective heat transfer coefficient measurements as a function of temperature. Characterized by 6 piecewise function, the average error is less than 1.20%.

The invention is not limited to the foregoing embodiments. The invention extends to any novel feature or any novel combination of features disclosed in this specification and any novel method or process steps or any novel combination of features disclosed.

Claims (1)

1. A rapid measurement method for convective heat transfer coefficient is characterized by comprising the following steps:

the method comprises the following steps: based on a calibration experiment, acquiring the relation between the ultrasonic propagation speed V and the temperature T inside the tested piece;

step two: heating the test piece by ultrasonic pulse echo method_iTime of ultrasonic wave propagation time t_i,exp；

Step three: converting the measurement of the convective heat transfer coefficient into an optimization problem for solving unknown parameters of the boundary of the heat conduction problem; the following optimization objective function is used:

wherein: h is_cThe convective heat transfer coefficient is measured on the surface of the medium; t is t_i,calT calculated for a value_iThe ultrasonic wave propagation time of the moment, the measurement time sequence represented by subscript i, and N represents the total measurement time point number; l is the length of the test piece in the measured direction;

the constraint conditions are as follows:

wherein: k, C and rho are respectively the thermal conductivity, specific heat capacity and density of the material to be measured; t is_fThe temperature of the fluid medium is controlled by a heater; t is_tTemperature of the end for placing the ultrasonic probe, T₀Is the initial temperature of the test piece;

step four: solving the inverse problem of heat conduction by adopting an adjoint equation algorithm to obtain the convective heat transfer coefficient of the medium surface;

the solution process of the optimization algorithm comprises the following steps:

(1) inputting thermophysical property parameters, dimensions and initial values of the dielectric material;

(2) solving the heat conduction equation to obtain the temperature field T (x, T) and the target function J (h)_c) A value of (d);

(3) the adjoint equation algorithm is adopted to optimize the parameter value to obtain better h_c；

(4) Judging whether convergence is achieved (the convergence is less than or equal to 1e-6), and stopping calculation if the convergence is achieved; otherwise, returning to the step (2) to repeat iteration until convergence is reached;

(5) obtaining the convective heat transfer coefficient h of the medium surface_c。

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