Name:Azriel Varrand Khalevi

NRP: 5019211026

6-1C
Forced convection refers to the movement of fluids (such as air or water) caused by external
forces or pressure differences, such as fans, pumps or wind. In forced convection, the fluid
motion is driven by an external agent and can be controlled and directed.
On the other hand, natural convection refers to the fluid movement that occurs due to
temperature differences within the fluid itself. In natural convection, fluid motion is caused
by buoyant forces that arise due to density differences caused by temperature variations.
Convection caused by wind is an example of forced convection, because the movement of air
is driven by external forces, which in this case are pressure differences caused by
atmospheric conditions. Convection that occurs indoors due to hot radiators, for example, is
an example of natural convection, because air movement is driven by temperature differences
within the fluid itself.

6-2C
External forced convection refers to the movement of fluid that is caused by an external
force, such as a fan or pump, which generates a pressure difference to drive the fluid flow.
This type of convection occurs on the outer surface of an object when a fluid is forced to flow
over it.
Internal forced convection, on the other hand, occurs within a fluid when the fluid is forced to
flow through a pipe, duct, or other enclosed space by an external force, such as a pump or
compressor. This type of convection is commonly observed in cooling or heating systems,
where a fluid such as water or air is circulated through pipes or ducts to transfer heat.
A heat transfer system can involve both internal and external convection at the same time.
For example, in a car's engine cooling system, the coolant is circulated by an internal pump
through the engine block, where it absorbs heat. The hot coolant then flows through an
external radiator, where it is cooled by forced convection caused by a fan. The cooled coolant
is then circulated back to the engine block, where the cycle begins again. In this system, both
internal and external forced convection are involved in the transfer of heat from the engine to
the environment.
6-3C
The convective heat transfer coefficient is typically higher in forced convection compared to
natural convection. This is because in forced convection, the fluid motion is driven by an
external force, such as a pump or fan, which creates a higher velocity and turbulence in the
fluid. The higher fluid velocity and turbulence increase the rate of heat transfer from the
surface to the fluid, which results in a higher convective heat transfer coefficient.
In natural convection, the fluid motion is driven by buoyancy forces, which are much weaker
than the external forces present in forced convection. As a result, the fluid velocity and
turbulence in natural convection are typically lower, which leads to a lower convective heat
transfer coefficient.
However, it is important to note that the convective heat transfer coefficient can also be
affected by other factors such as the fluid properties, the geometry of the surface, and the
temperature difference between the surface and the fluid. Therefore, the actual convective
heat transfer coefficient in a specific situation depends on various factors and cannot be
generalized solely based on whether the convection is forced or natural.

6-4C
Blowing warm air from our lungs onto hot baked potatoes will cool it faster than letting it
cool naturally in the cooler air in the room. This is because blowing air over the surface of the
potato increases the rate of convective heat transfer between the potato and the surrounding
air.
When we blow air onto the potato, we create forced convection, which increases the velocity
and turbulence of the air near the potato's surface. This higher velocity air removes heat from
the potato's surface more rapidly than the cooler, still air in the room would, thereby
accelerating the cooling process.
Additionally, blowing on the potato may also increase the rate of evaporative cooling, as
some moisture on the potato's surface may be evaporated by the moving air. Evaporative
cooling is another heat transfer mechanism that removes heat from the surface of an object.
Overall, blowing warm air from our lungs onto a hot baked potato will increase the rate of
heat transfer from the potato to the surrounding air, which will result in a faster cooling rate
compared to letting it cool naturally in the cooler air in the room.
6-5C
The Nusselt number (Nu) is a dimensionless number that describes the convective heat
transfer between a fluid and a solid surface. It is a measure of the ratio of convective to
conductive heat transfer across a boundary layer, which is the thin layer of fluid adjacent to
the surface where velocity and temperature gradients exist.
The physical significance of the Nusselt number is that it relates the convective heat transfer
rate to the thermal conductivity of the fluid and the size and shape of the solid surface. In
other words, the Nusselt number indicates how efficient the convective heat transfer is,
relative to conductive heat transfer, in a given system.
The Nusselt number is defined as:
Nu = hL/k
Where h is the convective heat transfer coefficient, L is a characteristic length scale (such as
the diameter of a cylinder or the width of a plate), and k is the thermal conductivity of the
fluid.
The Nusselt number can be used to predict the heat transfer rate in a variety of engineering
applications, such as heat exchangers, cooling electronic devices, and internal combustion
engines. The value of the Nusselt number is affected by factors such as fluid velocity,
temperature difference, and the geometry of the surface, and therefore it is important to
consider these factors when analyzing convective heat transfer in a given system.

6-6C
Conduction is the transfer of heat through a solid material or a stationary fluid by molecular
diffusion. In a fluid, conduction is dominant when the temperature gradient is small and the
fluid is not in motion.
Convection is the transfer of heat through a moving fluid by the combined mechanisms of
advection (bulk motion of the fluid) and diffusion (molecular motion of the fluid).
Convection is dominant when the fluid is in motion and the temperature gradients are large.
The rate of heat transfer is generally higher in convection than in conduction because the
fluid motion in convection enhances the transfer of heat between the solid surface and the
fluid. The convective heat transfer coefficient is a measure of the efficiency of heat transfer
through convection, while the thermal conductivity of the fluid is a measure of its ability to
conduct heat through molecular diffusion.
The convective heat transfer coefficient takes into account the effects of the fluid motion,
turbulence, and the geometry of the surface, which can significantly increase the rate of heat
transfer compared to pure conduction. The thermal conductivity of the fluid, on the other
hand, only takes into account the material properties of the fluid and its ability to conduct
heat through molecular diffusion.
In summary, heat transfer through a fluid is dominant when the temperature gradient is small
and the fluid is not in motion, and convection is dominant when the fluid is in motion and the
temperature gradient is large. The rate of heat transfer is generally higher in convection due
to the enhanced transfer of heat caused by the fluid motion, and the convective heat transfer
coefficient differs from the thermal conductivity of the fluid because it takes into account the
effects of fluid motion and surface geometry on heat transfers.

6-7C
Incompressible flow refers to a fluid flow in which the density of the fluid is assumed to
remain constant throughout the flow. This is a good approximation for liquids and gases at
low speeds, where the change in density due to changes in pressure and temperature is
negligible. Incompressible flow is a fundamental assumption in many fluid mechanics
applications, such as in the design of pipelines, pumps, and turbines.
An incompressible fluid is a fluid that does not experience significant changes in density due
to changes in pressure and temperature. This means that its volume does not change
appreciably under compression or expansion. Examples of incompressible fluids include
liquids, such as water and oil, and gases at low speeds, such as air at atmospheric pressure.
The flow of a compressible fluid does not necessarily have to be treated as compressible. If
the Mach number of the fluid is low (i.e., the speed of the fluid is much lower than the speed
of sound), the change in density due to changes in pressure and temperature can be neglected,
and the fluid can be treated as incompressible . However, as the speed of the fluid approaches
the speed of sound, the change in density becomes significant, and the compressibility of the
fluid cannot be ignored. In such cases, the flow must be treated as compressible, and the
compressibility effects, such as shock waves and changes in pressure and temperature, must
be taken into account.

EXAMPLE:
The flow of oil in a journal bearing can be approximated as parallel flow between two large
plates with one moving plate and the other stationary. Such flows are known as Couette
flows. Consider two large isothermal plates separated by 2-mm-thick oil film. The the upper
plates move at a constant velocity of 6 m/s, while the lower plates are stationary. Both plates
are maintained at 15˚C. (a) Obtain relations for the velocity and temperature distributions in
the oil. (b) Determine the maximum temperature in the oil and the heat flux from the oil to
each plate

ANSWER: