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# Strategically Preparing Thermodynamics for GATE

Rishabh Bhadauria will share the right way to prepare Thermodynamics for GATE Mechanical. Rishabh secured AIR 130 in GATE 2015 (ME) and is currently working in BPCL.

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In this blog I will be telling all the Mechanical branch students how to prepare thermodynamics subject for GATE. As all the mechanical branch aspirants will be aware that Thermodynamics is one of the most important subject for mechanical branch. It is one of the core subjects and the concepts should be clear to prepare for other subjects as well. Other subjects like Refrigeration & air conditioning, Heat and mass transfer and even fluid mechanics also rely on the basics of thermodynamics.

So, now question comes that how do you prepare it? How much time to dedicate? What books to use? Which notes to be used? How and how much to practice? And few other questions. I will be answering all these.

#### Stats from last year GATE papers :-

 Year 2017 2016 2015 2014 2013 2012 2011 2010 Thermodynamics & its applications Total Marks 9 10 10 10 10 13 20 14

The above table shows the marks distribution for GATE paper. It can be seen that the range of marks for this subject varies from 9 to 20, average being 12 which is 12% of total marks. Obviously the marks distribution also shows, it is one of the most important subject for scoring well in GATE.

Now let us see how to do the preparation

## Books required for thermodynamics preparation:-

Thermodynamics – by P K Nag

Thermodynamics – by Cengel & Boles (Just for occasional reference)

RAC – by Khurmi

Notes – KRASH (ME), postal study course

## Time required for thermodynamics preparation:-

25 to 30 days (if you have 8 to 10 months of preparation) with 2 to 3 hours daily

10 to 15 days (if you have 4 to 5 months of preparation) with 4 to 6 hours daily

## Topics :-

1. Basic concepts
2. Energy interaction*
3. First law of Thermodynamics*
4. First law applied to flow process*
5. Second law of Thermodynamics*
6. Entropy*
7. Available energy, availability and Irreversibility
8. Properties of pure substance*
9. Properties of Gas and Gas mixture*
10. Thermodynamic relations, equilibrium and stability
11. Application (1) – Different cycles*
12. Application (2) – air conditioning*
13. Refrigeration*

The topics marked with * are more important and you have to solve more numericals in these topics.

Make sure that you have P K Nag as it will help with theory as well as numericals. Keep the notes with you as well. I would suggest keeping an extra note book which you should keep for preparing notes, different kind of notes, where you note down anything which you understand in concepts and formulae. Also keep one separate notebook for solving numericals. I would advise you to solve full length numericals wherever I suggest. If you keep full length numericals solved in a notebook that will help you later on while revising.

Now I will be telling topic wise how to prepare.

1. Basic concepts (1 day) – You will find it in starting chapters of P K Nag. The theory part is necessary for going further ahead with the preparations. Focus only on theory. Solve 5 to 10 solved numericals from P K Nag. That much will be enough.

1. Energy interaction (1 day) – This topic covers the basic different kind of processes like isothermal process, isobaric process etc and how to calculate different kind of values like Heat transfer, Work done etc. Generally in GATE at least 1 question will be there from this topic and since the topic is easy, there can be a conceptual type of question from this topic. This topic actually complements with other later topics, so it’s important to be strong in this topic. Solve all the solved numericals from P K Nag along with 30% unsolved numericals.

1. First law of Thermodynamics (2 days) – It is just the extension of last topic with some additional equations. It’s important to remember formulae from this topic and practice a lot questions. In GATE, numericals are based on the concept on the laws of thermodynamics. At least 1 numerical is always there from this topic. Again, solve all the solved numericals from P K Nag with 30% unsolved questions.

1. First law applied to flow process (1 day) – This topic covers the steady flow process (open and closed). One numerical is always there in GATE form this topic and it is very easy. The question from this topic is not conceptual but only required formula to be used. You should be aware what values to put where and that’s it. There are some small topics like Free expansion which you shouldn’t leave. You just need to be aware what values are to be taken in these processes. Again, solve all the solved numericals from P K Nag with 30% unsolved questions.

1. Second law of Thermodynamics (3 days) – There is a little bit of theory in this topic as well. Cover the theory part first and then go to numericals. The numericals are based on concept of Heat engines, Refrigerator and Carnot cycles. Once you understand the concept, the numericals are very to solve. You should be aware of how thermal sources or reservoirs are connected in series and parallel and how the calculation is done for these kind of questions. Generally 2 to 3 questions from this topic is there. Solve all the solved numericals and 20% of the unsolved numericals which will cover all kind of questions.

1. Entropy (2 days) – This topic is very important as it combines with other type of numericals. The numericals in this topic is formula based but you should be aware how the formula is generated which will help you understand the concept. Also, there are sometimes graph related questions in GATE. So, in theory part also cover the P-V and T-S diagrams. Solve all the solved numericals and 30% of unsolved numericals.

1. Available energy, Availability and Irreversibility (1 day) – Not so important topic in respect to GATE. There are sometimes question on Irreversibility. The concept is also a little confusing but once you understand it, you can solve any problems. Just solve all the solved numericals from P K Nag and that will be enough. There is some theory part as well. Don’t skip those.

1. Properties of pure substance (2 days) – It has a lot of theory part based on the concept. I advise you to refer Cengel & Boles if you are having difficulty in understanding the concepts. The topic parts like critical pressure, critical temperature, saturation state etc are very important. In GATE, sometimes they give extract of steam table to solve the questions so you have to practice those kind of numericals. Although the numericals are very simple, you should be aware of how to calculate all the values. Sometimes confusing problems are also there but if you cover all the solved numericals from P K Nag, it will cover all kinds of problems.

1. Properties of Gases and Gas mixture (2 days) – Just similar to last topic, the topic is not that big. It covers equations like ideal gas equation with formula based numericals. Normally the numericals from this topic are conceptual, so you need the cover the theory part more comprehensively. The numericals based on mixture of gases at times can be tricky, so I recommend solving all the solved problems and 20% unsolved problems from P K Nag.

1. Thermodynamic relations, equilibrium and stability (2 days) – The theory from this part is very important. All the TdS equations are required for solving majority of the problems. There are certain theorems and equations as well which you should cover as theory only. There are topics like Joule – Kelvin Effect, Isothermal compressibility etc which you won’t find in P K Nag along with other topics. They are towards the end section of the book. So, make sure you are not missing out on those. Covering only theory part from this section is enough.

1. Application (1) – Different cycles (6 days) – It is one of the most important topics of thermodynamics, There may be as much as 4 questions from this topic. You have to cover all kind of cycles – from Otto, diesel cycle to Brayton, Rankine and other cycles. You should be aware of P-V and T-S diagram of each and every cycle. In some cycles different kind of graph is required, so cover that accordingly. This topic is a little spread in P K Nag, so you have to cover the topics after searching them. Solve all the solved numericals along with 30% of unsolved numericals. You should be aware of what regeneration and reheating is. There are some direct formulae as well but you should be aware of how to reach to that formula. Also, calculation of efficiency, COP etc by using simple PV equations is generally required. It is a broad topic and required solving a lot of problems covering all types. Try to also cover steam turbines and nozzles along with this topic. Theory like velocity compounding and pressure compounding are not along with other topics. So, search for that and cover it.

1. Application (2) – Air conditioning (3 days) – This is a totally different kind of topic. I suggest you to prepare this topic either from Khurmi or P K Nag. You should be aware of psychrometric chart in detail. There are different values like DBT, DPT, WBT etc and different formulae for all these values. Also, it cover different kind of processes like sensible heating, sensible cooling etc. Normally the questions are very conceptual from this topic so you need to practice as many numericals as possible. There are topics are by-pass factor which you should cover by Khurmi. I would suggest you to solve all the solved numericals from P K Nag as well as Khurmi.

1. Refrigeration (3 days) – Cover this topic from Khurmi. There are different kind of refrigeration cycles and some theory part like effect of decrease or increase evaporator pressure or effect of super-heating etc. The numericals are of various types covering from simple type of cycle to cascading, vapour absorption cycle etc. You need to practice all kinds of numericals. There are a lots of solved numericals in Khurmi which should be enough for practicing. Don’t forget about gas absorption cycle and theory part of different types of refrigerants as well.

Now that I have told about all the topics there certain other things to be taken care as well, which are:-

1. Cover the theory part from P K Nag but if you are having doubt in any topic, clarify it through Cengel and Boles. This book is a novel in itself. Any concepts can be made cleared using this book.
2. P K Nag covers a very large number of numericals, both solved and unsolved. Cover the number of questions that I mentioned here which will be more than enough. If you hung up on s single topic, it may become just a waste of time.
3. I would recommend solving full length numericals in a separate notebook as I mentioned earlier. This will not only help with accuracy but also with your revision, later on.
4. The timeline that I mentioned should be enough. You should keep that as target and complete the topics in mentioned timeline. Divide it into theory and numerical and cover as much as possible.
5. If you lag at any particular topic, don’t worry, try to make up with other topics.
6. If you are within target period, you should cover all the topics in 27 days.

#### Revision and practice –

Once you cover all the topics and numericals. You can go ahead with solving the last year papers questions for practice. There may be some questions which you may not be able to solve but if you are able to solve even 70% of problems, you are on right track. Take the target as 3 days to cover all the previous year questions papers.

If you have certain doubts on concepts while solving last year paper, you can take reference from your notes/books and make sure that you don’t make such mistakes again.

This covers the entire Thermodynamics subject in 30 days. If you have 8 to 10 months of preparations, this is the optimum time to dedicate in this subject.

If you are starting late and want to cover the subject fast, try to cover the mentioned timeline by divide it by 2 and dedicating more time each day. You can cut down on some numericals and make that happen.

If you are starting early, I would highly recommend reading Cengel and Boles just like a novel. It’s a book worth reading and I am sure you will like it. You will be very strong conceptually if you do that.

Good luck!