Aktu Btech Heat and Mass Transfer KME-501 Short Question

Provide the Heat and Mass Transfer Short Question Notes from the B.Tech. AKTU Quantum Book. Investigate the fundamentals of conduction, convection, and radiation as heat transport methods for effective energy use.

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Unit-I Introduction to Heat Transfer (Short Question)

Q1. What is the significance of heat transfer?

Ans. The discipline of the heat transfer encompasses a great many fascinating areas like 

• 1. The proper operation of valves and other controls that are activated by temperature changes, as well as thermal management of space vehicles.
• 2. Refrigeration and air conditioning systems, as well as a variety of additional cooling and heating devices. 

Q2. Discuss the mechanism of thermal conduction in gases, liquids and solids.

Ans.

• 1. Thermal conduction in solids occurs via lattice vibration and free electron transport.
• 2. In the case of gases, thermal conduction occurs as a result of molecules transferring energy and momentum.
• 3. The heat mechanism in liquids is more similar to that of gases. Yet, because the molecules are closer together, intermolecular forces become active.

Q3. What are free convection and forced convection?

Ans. When the fluid circulates due to the inherent difference in densities of hot and cold fluids, this is referred to as free or natural convection. Forced convection occurs when work is done to blow or pump the fluid.  

Q4. What do you mean by thermal conductivity ? Why good electrical conductor materials are also good heat conductors ?

Ans. Thermal Conductivity: It is a physical attribute of a substance and is defined as a substance’s ability to transfer heat through it. It is commonly symbolized by the letter k, and its unit is W/m-K.

Good Electrical Conductor Materials are also Good Heat Conductors: Because the ratio of thermal and electrical conductivities in all metals is precisely proportional to absolute temperature, materials that are good conductors of electricity are also good conductors of heat.

Q5. Write down the effect of temperature and pressure on thermal conductivity. 

Ans. Effect of Temperature on Thermal Conductivity: 

Thermal conductivity decreases with increasing temperature in solids and liquids, whereas thermal conductivity increases with increasing temperature in gases. 

Effect of Pressure on Thermal Conductivity: 

The thermal conductivity of a fluid or substance increases as pressure increases.

Q6. What do you mean by convective heat transfer coefficient? 

Ans. The amount of heat conveyed to a unit temperature difference between the fluid and a unit area of surface in unit time is defined as the convective heat transfer coefficient. It is commonly indicated by h and has the unit m²-K. 

Q7. What is the physical significance of thermal diffusivity?

Ans. The physical relevance of thermal diffusivity is that it tells us how quickly heat propagates or diffuses across a material as temperature changes over time. 

Q8. What do you mean by thermal contact resistance ? 

Ans. Thermal contact resistance is a thermal resistance to heat movement at the interface of two materials caused by surface roughness and blank areas.

Q9. What do you understand by overall heat transfer coefficient? 

Ans. The heat transmitted per unit area per unit time per degree temperature differential between the bulk fluids on each side of the substance is defined as the overall heat transfer coefficient.

Q10. What do you mean by critical thickness of insulation ?

Ans. The thickness of insulation up to which heat flow increases and then declines is referred to as the critical thickness of insulation. It is known as critical radius in the case of cylinders and spheres.

Q11. Describe the modes of heat transfer for heat loss from a thermos flask. 

Ans.

• 1. Heat loss through convection from the fluid to the inner surface of the flask.
• 2. Heat loss by conduction from the inner surface of the flask to the outermost surface of the flask.
• 3. Radiation heat loss from the flask’s outside surface to the atmosphere. 

Q12. What do you mean by thermal capacity ?

Ans. Thermal capacity is the amount of heat that material can hold. 

Mathematically,

Q13. What is the practical application of critical radius of insulation ? 

Ans. Following are the applications of critical radius of insulation 

• 1. Insulation of electric cables, and 
• 2. In steam and refrigeration pipes. 

Q14. Why are heat radiations invisible to the eye ?

Ans. Heat radiations have a longer wavelength than light waves, making them undetectable to the naked eye. 

Q15. Why does thermal conductivity exist in fluids ?

Ans. Thermal conductivity exists in fluid due to lattice vibrational waves. 

Q16. Why does the value of thermal conductivity tend to decrease with temperature in case of liquids ?  

Ans. In the case of liquids, the value of thermal conductivity tends to decrease with increasing temperature (with the exception of water) due to a decrease in density with increasing temperature. 

Unit-II Fins and Transient Conduction (Short Question)

Q1. What do you mean by fins ?

Ans. Fins are basically expanded surfaces that are used to increase the rate of transfer between the surface and the adjacent fluid by increasing the effective surface area.

Q2. Explain effectiveness and efficiency of fin. 

Ans. Fin Effectiveness: It is defined as the ratio of heat transfer rate with fin to the heat transfer rate without fin.

Efficiency of Fin (η_fin): It is defined as the ratio of actual heat transferred by fin to 10aximum heat transferable by fin, if entire fin area were at base temperature. 

Q3. Write down the conditions for fins to be effective. 

Ans. Following are the conditions for fins to be effective: 

• 1. Thermal conductivity (k) should be large. 
• 2. Heat transfer coefficient (h) should be small. 
• 3. Thickness of the fin should be small.  

Q4. Explain the significance of Heisler’s charts. 

Ans.

• 1. From Heisler chart, the value of B_i (Biot number) and F₀(Fourier number) are calculated on the basis of characteristic parameter S which is semi-thickness in case of plates and surface radius in case of cylinder and spheres. 
• 2. Heisler charts are extensively used to determine the temperature distribution. 

Q5. State the assumptions made in lumped heat capacity method for analysis of transient heat conduction problem. 

Ans. Assumptions made in lumped heat capacity method for analysis of transient heat conduction are as follows 

• 1. Internal resistance is insignificant in compared to surface resistance.
• 2. At any one time, the temperature across the solid is regarded to be uniform.

Q6. What do you mean by Biot number ?

Ans. Biot number is defined as the ratio of internal (conduction) resistance to surface (convection) resistance. 

Q7. What do you mean by Fourier number ?

Ans. The non-dimensional number

 is called the Fourier number. It signifies the degree of penetration of heating or cooling effect through a solid. 

Q8. What is the criteria of lumped heat capacity analysis ?

Ans. IfBiot number (Bi) < 0.1, the lumped heat capacity approach can be used to advantage with simple shapes such as plates, cylinders, spheres and cubes. 

Q9. Define response time of a thermocouple. What does it signify ?

Ans. Response Time of Thermocouple: It is defined as the time required for the thermocouple to attain the source temperature.

Q10. Define sensitivity. 

Ans. Sensitivity is defined as the time required by a thermocouple to reach its sensitivity of 63.2% of the original temperature difference. 

Q11. Why are thin and closely spaced fins always preferred? 

Ans. Since, 

If the ratio of P(perimeter) and A (cross-sectional area) is increased the effectiveness of the fin is improved. Due to this reason, thin and closely spaced fins are preferred. 

Unit-III Forced & Natural Convection (Short Question)

Q1. Define thermal boundary layer thickness.  

Ans. The thickness of the thermal boundary layer is defined as the distance away from the surface where the temperature difference is 99% of the temperature difference between the fluid’s surface and free stream temperature. 

Q2. What do you mean by boundary layer? 

Ans. The boundary layer is the layer of fluid adjacent to the border. When there is relative motion between the boundary and the fluid, a boundary layer forms.

Q3. Write the assumptions made for momentum equation for hydrodynamic boundary layer. 

Ans. Following are the assumptions: 

• 1. The flow is steady and incompressible. 
• 2. The viscosity of fluid is constant. 
• 3. Viscous shear forces in the Y-direction are negligible. 
• 4. Fluid is continuous in space and time.  

Q4. Define Prandtl number. 

Ans. Prandtl number is defined as the ratio of kinematic viscosity (v)to thermal diffusivity (α). Mathematically,

Q5. What do you mean by Grashof number ? 

Ans. The Grashof number is defined as the product of inertia and buoyancy forces divided by the square of viscous force. 

Mathematically, 

Q6. Write down the significance of Reynold’s and Nusselt number.

Ans. Significance of Reynold’s Number: The bigger the value of Re, the greater the relative contribution of the inertia effect. The bigger the relative magnitude of viscous stresses, the smaller the value of Re.

Significance of Nusselt Number: The Nusselt number is an easy way to calculate the convective heat transfer coefficient. The convective heat transfer coefficient is directly related to the thermal conductivity of the fluid and inversely proportional to the relevant length parameter for a given Nusselt number.  

Q7. What do you mean by edge of the boundary layer ?

Ans. The boundary layer edge is the location in the boundary layer when the local fluid velocity is 99% of the free stream velocity. 

Q8. What do you mean by hydrodynamic entry length and fully developed region ?

Ans. Hydrodynamic Entry Length: It is the section of the tube where the boundary layer increases and the velocity distribution changes with length. 

Fully Developed Region: The fully developed region is the part of the tube where the velocity distribution does not change with length. 

Q9.  Write the examples of heat transfer by free convection.

Ans. Following are the examples of heat transfer by free convection: 

• 1. Heating of water in a pot by gas flame. 
• 2. Heating of room by a heater coil.
• 3. Cooling of electric motors, cylinders of automobile etc. 
• 4. Cooling of electronic equipments.  

Q10. Define volume coefficient of thermal expansion. 

Ans. The physical property denoting the thermal expansion of the is known fluid as volume coefficient of thermal expansion (β).

Q11. Why are turbulent boundary layers thicker than laminar boundary layers ?

Ans. Turbulent because turbulent boundary layers are thicker than laminar boundary layers and the velocity distribution is significantly more uniform due to fluid particle intermingling between different levels of the fluid.  

Q12. Why is heat transfer smaller in free convection as compared to forced convection?

Ans.  Heat transfer in free convection is substantially lower than in forced convection because flow velocity in free convection is much lower than in forced convection. 

Unit-IV Thermal Radiation (Short Question)

Q1. What do you mean by radiation heat transfer ?

Ans. Radiation heat transfer is defined as the transfer of energy across a system boundary induced only by a temperature differential via an electromagnetic mechanism. It does not necessitate the use of a medium.

Q2.  Write the factors upon which the rate of emission of radiation by a surface depends.  

Ans. Following are the factors : 

• 1. The temperature of the surface, 
• 2. The nature of the surface, and 
• 3. The wavelength or frequency of radiation.

Q3. What is intensity of radiation ? 

Ans. The rate of energy leaving a surface in a particular direction per unit solid angle per unit area of the emitting surface normal to the mean direction in space is described as radiation intensity.

Q4. Define emissivity. 

Ans. Emissivity (є) is described as a body’s surface’s ability to radiate heat. It is sometimes described as the ratio of a body’s emissive power to that of a black body of equal temperature.

Q5. Define the term “Irradiation” and “Radiosity”. 

Ans. Irradiation : It is defined as the total amount of radiation incident on a surface per unit time and unit area. It is measured in W/m².

Radiosity : It is used to indicate the total radiation leaving a surface per unit time per unit area.

Q6. State Wien’s displacement law

Ans. 

Q7. What do you mean by solid angle ?

Ans. A solid angle is a piece of space inside a sphere enclosed by a conical surface, with the vertex of the cone at the sphere’s centre.

Q8. A black body emits radiation of maximum intensity at wavelength of 0.5 m. Calculate its surface temperature and emissive power. 

Ans. 1. From Wien’s displacement law, 

Q9. What do you mean by reciprocity theorem ?  

Ans. According to the reciprocity theorem, the net radiant interchange can be calculated by estimating the one-way configuration factor from one surface to the other.

Mathematically, 

Q10. What is radiation shield ?

Ans. A radiation shield is a low-emissivity barrier wall placed between two surfaces to minimize radiation between them.

Q11. Write a short note on solar radiation. 

Ans. Solar radiation is the energy emitted by the sun’s many layers. Solar radiation is a function of the geometrical relationship between the earth’s surface and the sun, which changes on a daily and annual basis.

Q12. What is green house effect ?

Ans. The green house effect is the process through which radiative energy leaving a planetary surface is absorbed by green house gases such as carbon dioxide, methane, and water vapours. This energy is transferred to other components of the atmosphere and re-radiated in all directions. This energy transfer to the surface raises the temperature of the environment and contributes to global warming.  

Q14. Why does a concave surface have a shape factor with itself ?

Ans. When radiant energy from one portion of the surface is intercepted by the other half of the same surface, a concave surface has a form factor with itself. A surface’s shape factor with respect to itself is F_1-1.

Q15. Why a flat or convex surface has a zero shape factor with respect to itself ?

Ans.  The shape factor with regard to a flat or convex surface is zero (i.e., F_1-1 = 0). This is because any section of a flat or convex surface cannot see/view any other part of the same surface.

Q16. Why is the shape factor of a convex surface always unity with its enclosure ?

Ans. Because all of the heat radiated from a convex surface is intercepted by its enclosure, the form factor of a convex surface with its enclosure is always unity. 

Unit-V Heat Exchanger (Short Question)

Q1. What do you mean by heat exchanger ?

Ans. A heat exchanger is a piece of equipment that transfers energy from a hot fluid to a cool fluid at the fastest possible pace while requiring the least amount of investment and running costs.

Q2. How heat exchangers are classified?  

Ans. Heat exchangers are classified as follows:

• a. Classification According to Nature of Heat Exchanger: 
  • 1. Direct contact heat exchanger. 
  • 2. Indirect contact heat exchanger. 
• b. Classification According to Relative Direction of Fluid: 
  • 1. Parallel flow. 
  • 2. Counter flow. 
  • 3. Cross flow.
• c. Classification According to Design and Constructional Features: 
  • 1. Concentric tubes. 
  • 2. Shell and tube. 
  • 3. Multiple shell and tube passes. 
• d Classification According to Physical State of Fluids: 
  • 1. Evaporator. 
  • 2. Condenser.  

Q3. Counter flow heat exchanger is most preferred. Why?

Ans. Because the LMTD of a counterflow unit is always greater than that of a parallel flow unit, a counterflow heat exchanger can transmit more heat than a parallel flow heat exchanger. As a result, the counter flow configuration is commonly used. 

Q4. What do you mean by logarithmic mean temperature difference (LMTD) ? 

Ans. LMTD is defined as the temperature difference that, if constant, would result in the same rate of heat transfer as occurs under variable temperature difference conditions.

Q5. How can you prevent the fouling ?

Ans. The following methods may be used to keep fouling minimum : 

• 1. Design of heat exchanger, 
• 2. Treatment of process system, and 
• 3. By using cleaning system. 

Q6. What do you mean by fouling factor in analysis of heat exchanger ?

Ans. The reciprocal of scale heat transfer coefficient is called fouling factor.

Q7. Define capacity ratio.  

Ans. The capacity ratio M is the product of the mass flow rate and the heat capacity fluids. 

Mathematically, 

Q8. What do you mean by number of transfer unit ?

Ans.  The grouping of the term (UA)/C_miv is a dimensionless expression called the number of transfer units (NTU). It is a measure of effectiveness of the heat exchanger. 

Q9. Define condensation and their types.

Ans. Condensation: It is defined as the process through which a substance’s vapour phase transforms to liquid phase by releasing its latent heat of vapourisation.

Types of Condensation: 

• 1. Filmwise condensation. 
• 2. Dropwise condensation. 

Q10. What do you mean by boiling ?

Ans. Boiling is a process of convective heat transfer that involves a phase shift from liquid to vapour. Evaporation at a solid-liquid interface is another term for it. 

Q11. What is the difference between pool boiling and low boiling ? 

Ans. 

Q12. What are the uses of heat pipe ?

Ans.

• 1. Spacecraft, 
• 2. Cooking 
• 3. Permafrost cooling, and 
• 4. Computer system, etc.

Q13. Write the limitations of Fick’s law. 

Ans.

• 1. Only when mass diffusion occurs is Fick’s expression valid.
• 2. The mass transfer mechanism in gases is much faster than in liquids and much slower in solids.
• 3. The mass transfer process is highly dependent on molecule spacing and velocity.
• 4. Apart for the concentration gradient, it does not account the temperature gradient, pressure gradient, or any other internal for effect. 

Q14. Why is a counter flow more advantageous for a gas turbine heat exchanger ?

Ans. Parallel flow heat exchangers have a maximum efficiency of 50%. This maximum is 100% for counter flow. As a result, for gas turbine heat exchangers, a counter flow is usually more favourable.

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