• Anand Zambare

Answers to Thermofluid interview questions & some tips.

Thermodynamics, fluid dynamics & heat transfer. What are the must know things before an interview?


Fluid dynamics and heat transfer go hand in hand as the heat transfer is nothing but flow of heat. There are many concepts which fall under this roof. So what are those questions? Check out some interesting and must know questions to ace your next interview.


1. Types of heat transfer?

(This is a very basic question, to begin with. We can expect more discussion on heat transfer after this question.) Generally, heat transfer is by three ways a) Conduction b) Convection and c) Radiation.


2. What is a heat exchanger? Types of heat exchangers?

(This is a specific question by process industries) The heat exchanger is a device that is used for exchanging heat over some length and time scale. Depending on how cold and hot fluids travel in the heat exchangers, there are two types of heat exchangers a) Crossflow and b) Parallel flow heat exchangers. Depending upon the geometry also there are types like shell and tube or plate heat exchangers and so on.


3. Which type of heat transfer takes place in a heat exchanger?

(This question can be asked as a continuation of the previous question) Inside the heat exchanger overall there will be conjugate heat transfer. Conjugate means, first of all there will be heat transfer between the fluid (hot) to the pipe which is carrying that through convection. After that, inside the pipe, there will be pure conduction heat transfer. After conduction inside the pipe, from the outermost surface of the pipe, again convection will take place and there will be heat transfer to cold fluid (In case of direct heat transfer) or pipe surface which carries cold fluid.


4. What is the effectiveness of a heat exchanger?

(This is a very typical question in the heat transfer subject. It can be asked in the first go itself, if we mention heat transfer as one of our favorite subjects.) The effectiveness of a heat exchanger is defined as the ratio of actual heat transfer over the maximum possible heat transfer. There are two methods using which generally, the effectiveness of the heat exchanger is measured. NTU method and LMTD method. NTU stands for No. of Transfer Units (NTU). LMTD stands for Log Mean Temperature Distribution (LMTD). Generally, the NTU method is preferred over the other method for calculating effectiveness.


5. How is the heat exchanger affected by fouling?

The fouling effect is nothing but the deposition of the impurities inside the pipes of a heat exchanger. The deposition comes from the impurities inside the fluid which is used in the process. Deposition leads to more resistance in the pipes for heat exchange and can cause some severe damage. The following factors can be the cause for such impurities to get deposited.

a. Low wall shear stress

b. Low velocity of fluids.

c. Precipitation of impurities due to the high temperatures.

d. Product solid precipitation.

This should be avoided for maintaining the effectiveness of the heat exchanger. For that maintenance is to be carried out periodically.


6. What is a parallel-flow heat exchanger and cross-flow heat exchanger?

Heat exchangers are typically classified according to flow arrangement and type of construction. The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions. This heat exchanger consists of two concentric pipes of different diameters. In the parallel-flow arrangement, the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end. In the counter-flow arrangement, the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends. The design of a parallel flow heat exchanger is advantageous when two fluids are required to be brought to nearly the same temperature.


7. What is the LMTD method?

The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred. The use of the LMTD arises, straightforwardly, from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.


8. What is the NTU method?

When direct knowledge of the LMTD is not available and the NTU method (Number of Transfer Units method) can be used. This method is based on a dimensionless parameter called heat transfer effectiveness. The effectiveness is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate. To define the effectiveness of a heat exchanger, we must first determine the maximum possible heat transfer rate for the heat exchanger.


9. Basics of heat transfer particularly convection.

Convection heat transfer is nothing but the combination of conduction and advection. Advection is caused by the bulk motion of a fluid (driven by any force say gravity/buoyancy). There are a lot of factors influencing the convection heat transfer process. Some important factors are Flow regime, type of fluid, surface roughness, etc. Analytically the convection heat transfer is given by surface area times the temperature difference times the convection coefficient. Determining the convection coefficient is a tedious job and takes a lot of maths behind it.


10. What are the Laws of thermodynamics?

The zeroth law of thermodynamics defines thermal equilibrium and forms a basis for the definition of temperature: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. The first law of thermodynamics states that, when energy passes into or out of a system (as work, heat, or matter), the system's internal energy changes in accord with the law of conservation of energy. The second law of thermodynamics states that in a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems never decreases. Another form of the statement is that heat does not spontaneously pass from a colder body to a warmer body. The third law of thermodynamics states that a system's entropy approaches a constant value as the temperature approaches absolute zero. With the exception of non-crystalline solids (glasses), the entropy of a system at absolute zero is typically close to zero. For more details please check out the following blog. https://www.mechnflow.com/post/thermodynamics-key-ingredients


11. Difference between Enthalpy and entropy.

Entropy is the measure of the thermal energy of a system per unit temperature. It is the measure of unavailable energy in a closed thermodynamic system and is concerned with measuring the molecular disorder, or randomness, of the molecules inside the system. In simple terms, entropy is the degree of disorder or uncertainty in the system. Enthalpy is a central factor in thermodynamics. It is the total heat contained in the system. This means if the energy is added, the enthalpy increases. If the energy is given off then the enthalpy of the system decreases. https://www.mechnflow.com/post/carnot-engine-cycle for more understanding of entropy please check.


12. What are the different types of boilers?

The boilers are classified using different parameters. Depending on the tubes the boilers are classified as Water tubes and Fire tubes. Depending on the no. of tubes boilers are classified as Single tube and multitubular boilers. Depending on the position of the furnace internally and externally fired boilers. According to the axis of the shell, vertical and horizontal boilers. Depending on the methods of circulation of water and steam, Natural circulation and forced circulation boilers.


13. If I have a rod free at both ends and I heat at its center, is thermal stress developed in the rod?

There will not be any stress generation in the rod. As the ends are free there will be some elongation of the rod without any opposition and hence the thermal stress generated would be zero.


14. Fluid Mechanics questions such as Navier stokes equations and methodological breakdown of a complex problem for analysis.

So for answering this question, we can first of all start with what Navier-Stokes questions are exactly. Navier-Stokes equations are the equation of motion which include Pressure force, Viscous force, and Gravity force in the equation. After this, we can write one component of the equation of motion. By stating the variables involved in it, we can elaborate on how one can solve the Navier-Stokes equation numerically using CFD. For more details, you should visit https://www.mechnflow.com/post/conservation-equations-in-fluid-dynamics


15. Derivation for convection formulae?

This is again a mathematical question and one can refer to any standard book of heat transfer for the derivation of convection formulae. Heat Transfer by Incropera is one of the best books to refer to for heat transfer.

 

These are just a few, there will be many more. To get a head start on your next interview and a heads up on our next post, subscribe to our blog. Till then, all the best!!

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