Forced Convection MCQ Quiz - Objective Question with Answer for Forced Convection - Download Free PDF

Explanation:

Forced Convection

• Forced convection is a mechanism of heat transfer in which a fluid (such as air, water, or any other liquid or gas) is forced to flow over a surface or through a channel by an external device, such as a fan, pump, or blower. The external device generates motion in the fluid, facilitating the transfer of heat between the surface and the fluid. Forced convection is widely employed in various industrial and engineering applications due to its efficiency in transferring heat over large surfaces or volumes.
• In forced convection, the rate of heat transfer is significantly higher compared to natural convection. This is because the external force increases the velocity of the fluid, reducing the thermal boundary layer thickness and enhancing the heat transfer coefficient.
• In forced convection, an external device (such as a fan, pump, or blower) is used to induce motion in the fluid, enhancing the heat transfer process. The external force overcomes the resistance of the fluid, ensuring a controlled and efficient heat transfer mechanism. This distinguishes forced convection from natural convection, where the fluid motion is driven by natural buoyancy forces. The heat transfer in forced convection follows Newton's law of cooling, which states:

Q = h × A × ΔT

Where:

• Q = Rate of heat transfer (W)
• h = Convective heat transfer coefficient (W/m²·K)
• A = Surface area of heat transfer (m²)
• ΔT = Temperature difference between the surface and the fluid (K)

Examples of Forced Convection:

• Air Conditioning Systems: Fans are used to circulate air, improving the transfer of heat between the air and the cooling/heating coils.
• Car Radiators: A pump circulates coolant through the engine and radiator, while a fan helps dissipate heat from the radiator to the surrounding air.
• Heat Exchangers: Pumps and blowers are employed to move fluids through the heat exchanger, enhancing the transfer of heat between the fluids.
• Electronics Cooling: Fans are used to cool electronic components by forcing air over heat sinks or circuit boards.

Explanation:

Forced Convection

Definition: Forced convection is a mode of heat transfer in which fluid motion is generated by an external source like a pump, fan, or a mixer. This movement enhances the heat transfer rate between a solid surface and the fluid or between different fluid layers. Unlike natural convection, where the fluid motion is driven by buoyancy forces due to density variations caused by temperature differences, forced convection relies on external mechanisms to create the fluid flow.

Working Principle: In forced convection, an external device such as a fan or pump moves the fluid over a surface, thereby increasing the heat transfer rate. The motion of the fluid disrupts the thermal boundary layer, which is the layer of fluid in the immediate vicinity of the heat transfer surface where the temperature gradient is significant. By reducing the thickness of this boundary layer, forced convection enhances the heat transfer coefficient, resulting in more efficient heat transfer.

Example: The correct example of forced convection from the given options is "Air blown over a car radiator by a fan" (Option 4). In this case, the fan forces air to flow over the radiator's surface, thereby increasing the heat transfer from the hot coolant within the radiator to the air. This forced movement of air significantly improves the cooling efficiency of the radiator.

Advantages:

• Higher heat transfer rates compared to natural convection due to increased fluid velocity.
• Better control over the heat transfer process since the fluid flow can be regulated by adjusting the speed of the external device (fan, pump, etc.).

Disadvantages:

• Requires additional energy input to operate the external devices such as fans or pumps.
• More complex system design and higher initial costs compared to natural convection systems.

Applications: Forced convection is widely used in various engineering applications where efficient heat transfer is crucial, such as in automotive cooling systems, air conditioning units, heat exchangers, and electronic device cooling.

Analysis of Other Options:

Option 1: Heat transfer through a stationary fluid layer

This option describes conduction rather than convection. In conduction, heat transfer occurs through a stationary medium (solid or fluid) by the transfer of energy from one molecule to another without any bulk movement of the medium itself.

Option 2: Thermal energy transmitted by electromagnetic waves

This option describes radiation, not convection. Radiation is a mode of heat transfer where thermal energy is transmitted in the form of electromagnetic waves (e.g., infrared radiation) and does not require a medium for transmission.

Option 3: Warm air naturally rising from a hot surface

This option describes natural convection. In natural convection, the fluid motion is caused by buoyancy forces that arise from density differences due to temperature gradients. Warm air rises naturally from a hot surface as it becomes less dense compared to the cooler surrounding air.

Option 5: [Blank]

No information is provided for option 5, so it cannot be evaluated.

Explanation:

Forced Convection

Definition: Forced convection is a mechanism or type of heat transfer in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). This differs from natural convection, where the fluid motion is caused by buoyancy forces that result from density variations due to temperature gradients in the fluid.

Working Principle: In forced convection, the external device (such as a fan or pump) actively moves the fluid, enhancing the heat transfer process. The movement of the fluid increases the rate at which heat is transferred from a surface to the fluid or from the fluid to a surface, depending on the temperature difference.

Advantages:

• Increased heat transfer rate compared to natural convection.
• Better control over the heat transfer process due to the ability to regulate the speed and direction of the fluid flow.
• Enhanced cooling or heating efficiency in various applications, such as electronic devices, industrial processes, and HVAC systems.

Disadvantages:

• Requires an external power source to operate the fan, pump, or other devices.
• Can be more complex and expensive to implement compared to natural convection systems.

Applications: Forced convection is widely used in many applications, including cooling of electronic components, HVAC systems, automotive cooling systems, and industrial heat exchangers.

Explanation:

In a thermally fully developed flow, the temperature profile will no longer change along the length of the flow, similar to the velocity profile. It means there is no further growth of the boundary layer. This is the case of a steady state.

 = Constant for thermally fully developed flow

The rate of increase in bulk mean temperature along the flow  increase exponentially along the flow direction (z).

Explanation:

Rayleigh Number:

• The Rayleigh number is a dimensionless number in fluid dynamics and heat transfer that is important in predicting the onset of convection.
• It characterizes the flow regime in a fluid and is particularly useful in free convection scenarios.
• The Rayleigh number combines the effects of thermal expansion, viscosity, thermal diffusivity, and the temperature gradient driving the convection.
• The Rayleigh number (Ra) is given by the product of the Grashof number (Gr) and the Prandtl number (Pr).

Mathematically, it is expressed as:

Ra = Gr × Pr

Where:

• Gr is the Grashof number, which represents the ratio of buoyancy to viscous forces in the fluid.
• Pr is the Prandtl number, which represents the ratio of momentum diffusivity (viscosity) to thermal diffusivity.

Working Principle: In a free convection scenario, fluid motion is induced by the buoyancy forces that result from density variations due to temperature gradients within the fluid. The Rayleigh number helps determine whether the fluid flow will remain laminar or transition to turbulence. A higher Rayleigh number indicates a greater likelihood of turbulent flow.

Importance: The Rayleigh number is critical for engineers and scientists to predict and analyze the thermal behavior of fluids in various applications such as heating and cooling systems, natural convection in the atmosphere, and geological processes.

Explanation:

Prandtl member is the ratio of momentum diffusivity to thermal diffusivity.

Typical ranges of Prandtl member is listed below

Concept:

Always remember standard results mentioned below;

Part – I For constant surface heat flux (q_s = constant);

Hydrodynamically and thermally fully developed laminar flow through a circular pipe of constant cross section

Nu_q = 4.36      …1)

Part – II For constant wall temperature (T_w = constant)

For this case

Nu_T = 3.66     …2)

Calculation:

Comparing 1) and 2)

Nu_q > Nu_T ⇒ Option A is correct.

Key Points

Go through both derivations (i.e. q_s = constant and T = constant) and remember graphs of both cases.

Concept:

Newton's law of cooling states that,

Where,

Q = Convective heat transfer

h = heat transfer coefficient

 = wall temperature

 = bulk mean temperature

Fig: Flow of fluid through a tube with constant wall temperature

• Since the existence of convective heat transfer between the surface and the fluid dictates that fluid temperature must continue to change with x.
• Using, we get the following temperature distribution for T_m

Fig: Variation of bulk mean temperature in the tube(constant wall temp)

• In the entrance region, the value of h varies along x and h has high value at the initial stage and later the value decreases as the flow is close to fully developed.

Fig: Variation of h along the length of the tube

• In a fully developed region, the value of h remains constant.

Convective mass transfer: The transport of material between a boundary surface and a moving fluid or between two immiscible moving fluids separated by a mobile interface is known as convective mass transfer. 

Sherwood number (Sh): The ratio of convective mass transfer rate to mass transfer by molecular diffusion is called Sherwood number. 

Schmidt number (Sc): The ratio of molecular diffusivity of momentum to molecular diffusivity of mass transfer is called Schmidt number. 

Rayleigh number: The product of Grashof number and Prandtl No is called Rayleigh number

Ra = Gr pr

Concept:

A coolant fluid flows over a heated flat plate means here we can assume that Natural Convection is equal to Conduction of heat through that plate, i.e. 

where, h = convective heat transfer coefficient, ΔT = temperature difference, k = thermal conductivity 

Calculation:

Given:

T_p = 100°C, T_∞ = 30°C, k = 1 W/mK, T = 30 + 70exp(-y)

Therefore, 

⇒ h × (100 - 30) = -1 × (-70)

∴ h = 1 W/m2K

Concept:

For solving this question, draw temperature (T) versus x profile.

Flow will be thermally fully developed when slopes of both mean fluid temperature line (T_m) and wall temperature line (T_s) are same.

Calculation:

Slope of line

Slope of line

⇒ Flow is not thermally developed at P.

Slope of line

Slope of line

⇒ Flow is developed thermally at point Q and R.

Alternate Method

In case of uniform heat bulk mean temperature varies linearly. The difference between bulk mean temperature and wall temperature is constant in thermally developed region.

so bulk temperature.

T(x) = A + Bx (i)

At x = 0, T = 0°C

A = 0°C       (ii)

At x = 1, T = 50°C

B = 50°C    (iii)

from (i), (ii) & (iii)

bulk temperature T(x) = 50x

So, Q and R in thermally developed region.

Concept:

Heat loss through the convection is given by

Q_loss = h × A × (∆T)

Nusselt number 

Where l­_c is the characteristic length, in case of heater it is equal to the diameter of the sphere

Calculation:

Given, Nu = 31.4, k = 0.026 W/mK, D = 12 mm ⇒ r = 6 × 10-3 m , Surface temperature T_s = 27°C and atmospheric temperature T_∞ = 77°C

⇒ 

Surface area of sphere A = 4 × π × r² = 4 × 3.14 × (6 × 10^-3)²

Q_loss = 68.03 × 4 × 3.14 × (6 × 10^-3)² × (77 – 27) = 1.54 J/s

Explanation:

Forced Convection

Definition: Forced convection is a mechanism or type of heat transfer in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). This differs from natural convection, where the fluid motion is caused by buoyancy forces that result from density variations due to temperature gradients in the fluid.

Working Principle: In forced convection, the external device (such as a fan or pump) actively moves the fluid, enhancing the heat transfer process. The movement of the fluid increases the rate at which heat is transferred from a surface to the fluid or from the fluid to a surface, depending on the temperature difference.

Advantages:

• Increased heat transfer rate compared to natural convection.
• Better control over the heat transfer process due to the ability to regulate the speed and direction of the fluid flow.
• Enhanced cooling or heating efficiency in various applications, such as electronic devices, industrial processes, and HVAC systems.

Disadvantages:

• Requires an external power source to operate the fan, pump, or other devices.
• Can be more complex and expensive to implement compared to natural convection systems.

Applications: Forced convection is widely used in many applications, including cooling of electronic components, HVAC systems, automotive cooling systems, and industrial heat exchangers.

Explanation:

Forced Convection

Definition: Forced convection is a mode of heat transfer in which fluid motion is generated by an external source like a pump, fan, or a mixer. This movement enhances the heat transfer rate between a solid surface and the fluid or between different fluid layers. Unlike natural convection, where the fluid motion is driven by buoyancy forces due to density variations caused by temperature differences, forced convection relies on external mechanisms to create the fluid flow.

Working Principle: In forced convection, an external device such as a fan or pump moves the fluid over a surface, thereby increasing the heat transfer rate. The motion of the fluid disrupts the thermal boundary layer, which is the layer of fluid in the immediate vicinity of the heat transfer surface where the temperature gradient is significant. By reducing the thickness of this boundary layer, forced convection enhances the heat transfer coefficient, resulting in more efficient heat transfer.

Example: The correct example of forced convection from the given options is "Air blown over a car radiator by a fan" (Option 4). In this case, the fan forces air to flow over the radiator's surface, thereby increasing the heat transfer from the hot coolant within the radiator to the air. This forced movement of air significantly improves the cooling efficiency of the radiator.

Advantages:

• Higher heat transfer rates compared to natural convection due to increased fluid velocity.
• Better control over the heat transfer process since the fluid flow can be regulated by adjusting the speed of the external device (fan, pump, etc.).

Disadvantages:

• Requires additional energy input to operate the external devices such as fans or pumps.
• More complex system design and higher initial costs compared to natural convection systems.

Applications: Forced convection is widely used in various engineering applications where efficient heat transfer is crucial, such as in automotive cooling systems, air conditioning units, heat exchangers, and electronic device cooling.

Analysis of Other Options:

Option 1: Heat transfer through a stationary fluid layer

This option describes conduction rather than convection. In conduction, heat transfer occurs through a stationary medium (solid or fluid) by the transfer of energy from one molecule to another without any bulk movement of the medium itself.

Option 2: Thermal energy transmitted by electromagnetic waves

This option describes radiation, not convection. Radiation is a mode of heat transfer where thermal energy is transmitted in the form of electromagnetic waves (e.g., infrared radiation) and does not require a medium for transmission.

Option 3: Warm air naturally rising from a hot surface

This option describes natural convection. In natural convection, the fluid motion is caused by buoyancy forces that arise from density differences due to temperature gradients. Warm air rises naturally from a hot surface as it becomes less dense compared to the cooler surrounding air.

Option 5: [Blank]

No information is provided for option 5, so it cannot be evaluated.

Concept:

Fourier number is a dimensionless group that arises naturally from the non – dimensionalization of the conduction equation. It is very widely used in the description and prediction of the temperature response of materials undergoing transient conductive heating or cooling.

Weber Number: It is defined as the ratio of the inertia force to surface tension force.

Grasshoff Number: It is the ratio of buoyant forces to the viscous forces. Grashof Number will perform the same function in natural convection as played by Reynold’s number in forced convection.

Schmidt Number: It is the ratio of momentum diffusivity and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusing convection processes.