/r/thermodynamics

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Here is a place to discuss the study and application of Thermodynamics and Heat Transfer, as relates to physics, cosmology, chemistry, engineering, materials science, earth sciences and biology. Thermodynamics deals with the relations between heat and other forms of energy (such as mechanical, electrical, or chemical), focused predominantly on equilibrium or quasi-equilibrium systems. Heat Transfer concerns the generation, use, conversion, and exchange of thermal energy between physical systems.


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/r/thermodynamics

11,054 Subscribers

2

The difference between ideal gas and non ideal gas?

Why is it so important that we use ideal gas conditions in thermodynamic processes? Compared to non-ideal gas behavior?

7 Comments
2023/06/04
11:42 UTC

0

Looking to model condensing flow through a packed bed (porous media) analytically. Numeric solutions for this exist but are computationally expensive. Can I reasonably use the paper linked in the description to model the condensation front (and temperature profiles for example) analytically?

https://link.springer.com/content/pdf/10.1007/s11242-004-5740-5.pdf

The paper above attempts to simplify the differential equations used to model condensing/evaporating flow through a packed bed. I am just wondering if equations 30 to 33 can be used to model 1) the condensation front position in the packed bed versus time/position and 2) the temperature profile in the packed bed versus time/position

Open to any discussion on finding analytic solutions to the movement of the condensation/evaporation front also!

0 Comments
2023/06/02
15:39 UTC

6

Why is it really cold in the mountains?

This is a question that has bothered me for a while. It sounds like a simple question, but it is actually not that trivial. If you look online you find a lot of different explanations, some of which are clearly wrong.

I did a lot of digging, and came up with a few simple interactive simulation models to illustrate some key concepts, that lead to cold mountains.

https://i.redd.it/es0ihokxou2b1.gif

In this simulation, for example, white dots represent visible light, and the orange dots represent heat radiation. The heat radiation is stochastically emitted based on the temperate of each slab, which is indicated by its color.

If you are interested, you can find the full story on my website:

https://marblescience.com/blog/why-is-it-really-cold-in-the-mountains

8 Comments
2023/05/29
19:06 UTC

3

Can hot air escape or vent downward?

I’m installing a 12000btu a/c in a party bus. I’m sealing it in the nose section of the coach which is over the cab of the truck where the driver sits. I want to put vents for dissipating the heat the unit creates in the areas where the coach over hangs above both truck doors. That would mean the heat would have to vent lower than it’s origin mm. Also while moving there would be road wind swirling outside the vents. Does that make sense? And any advice would be awesome.

2 Comments
2023/05/28
18:33 UTC

1

can you guys explain to me why is there this number popped up out of nowhere

11 Comments
2023/05/27
16:34 UTC

4

Am I on the right path to finding the final pressure and temperature of an ideal gas in a spinning body?

I'm facing a fascinating challenge that requires some thermodynamic calculations related to a continuous flow centrifuge <https://www.news-medical.net/whitepaper/20160926/Overview-of-Continuous-Flow-Centrifugation.aspx>. Before diving into the design and construction process, I need to determine the required radius and rate of spin to achieve specific target pressures and temperatures. While I have good building skills, my expertise in thermodynamics is limited. Therefore, I would greatly appreciate your assistance in working out the necessary formulas for this problem.

The objective at hand is to find the pressure and temperature at the final point [f], given the initial state. Let me provide you with the details of the apparatus and the parameters involved.

Simplified CV Overview

The apparatus involves the following elements: The centrifuge operates with one kilogram per second of air entering at the initial point [i]. It has an initial temperature of 293K and a pressure of 100,000Pa. The centrifuge spins around an axis at a rate of 10.47 rad/s (equivalent to 100 RPM). The distance between the axis and the center of the intake at point [i] is 0.2 meters, while the distance between the axis and point [f] is 1.0 meter. Beyond point [f], there is an expansion valve that limits the flow rate to 1 kilogram per second.

To simplify the calculations, I made the assumptions that we are using an ideal gas, that the process is adiabatic, the system is in a steady state, the system is isentropic, frictionless, and laminar flow.

Given the parameters involved, we need to solve for the final pressure and temperature at point [f]. I came up with the following formulas:

T_f = ((r^2 * ω^2) / (m_dot * Cp)) + T_i

P_f = P_i * Exp[((M * ω^2) / (2 * R * T)) * r^2]

Link to my work to get these formulas:

https://drive.google.com/drive/folders/1qYrg6nKM1A0nz5Ec_oO5HksEFkJRGzwz?usp=sharing

While I have tinkered with these formulas, I'm unsure if my approach is correct as it did not pass my gut-feeling test as the results do not look right at high rates of spin (>10,000 RPM). Your expertise and insights would be invaluable in tackling this problem effectively.

I apologize if this post resembles a homework problem, as it was not my intention. I genuinely seek guidance for a real-world centrifuge application and am feeling way out of my depth. Thank you in advance for any assistance you can provide!

Edit: It was mentioned that the apparatus description was not clear. I hope this clarification helps.

To simplify the setup, imagine a spinning disk with a pipe attached, resembling a spoke of a wheel. Air enters the pipe near the axle and moves through it, gradually increasing in pressure until it reaches the final point of interest, which is the farthest from the axle. Finally, the end of the pipe is equipped with an expansion valve that regulates the flow rate through the pipe to a predetermined level.

16 Comments
2023/05/27
09:31 UTC

2

Clues How to Determine Thermodynamic Process

Here's the problem: "Gas enclosed in a cylinder with a weighted piston as the top boundary. The gas is heated and expands from a volume of 0.04m3 to 0.10m3. The pressure varies such that PV=constant, and the initial pressure is 200kPa. Calculate the work done by the system."

Now, I'm not looking for the solution, I already have it. My question is why is this problem "isothermal"? What clues should I look for to determine such or better yet, a fundamental reasoning why this is isothermal. Hope you can help me.

4 Comments
2023/05/26
17:39 UTC

3

Getting a set temperature and pressure of an ideal gas mix in a fixed volume

I have equations that solve for the question, but I do not understand why they work, nor just exactly what mechanism is being used so that I could research it. The problem:

I have a fixed volume containing an ideal gas mixture, whos state is known. I wish to change both the temperature and the pressure of the gas to set values. I have two separate ideal gas mixtures of known state for inputs, one is at a high temperature and one is at a low temperature. The formula below determines how many moles from the hot and cold inputs must be mixed with the gas in the fixed container to meet my given set points.

Where xF stands for desired values, x1 stands for hot input, x2 stands for cold input, and x3 stands for gas already in the mixing chamber. N is moles of gas, C is specific heat of gas, T is temperature, and P is pressure.

V = 1000 , R = 8.3144

NF = (PF*V) / (R*TF)

N1 = ((C2*T2*NF)-(C2*T2*N3)+(C3*T3*N3)-(C2*TF*NF)+(C2*TF*N3)-(C3*TF*N3)) / ((TF*C1)-(TF*C2)-(T1*C1)+(T2*C2))

N2 = NF - (N1+N3)

NF and N2 are straightforward enough, but N1 has me puzzled. I think that I see a bunch of internal energy calculations in there?

6 Comments
2023/05/23
16:58 UTC

1

Formula for enthalpy?

Im new to this and i cant seem to find the different formulas for enthalpy for stationary and instationary processes. Can someone please help me?

12 Comments
2023/05/23
11:42 UTC

6

Boundary layers explain leaf size variation!

0 Comments
2023/05/21
01:35 UTC

5

Looking for any papers that model condensing flow of a gas through a packed bed (with some output that I can tune my model to). Have had no luck so far! Does anyone know of any? (more info in description)

I'm looking for any publication that involves passing a gas through a cold packed bed, and in the process condensing that gas, producing liquid. My model does this, however its pretty useless unless I can show that it produces realistic data!

The only paper I have found is this one: https://asmedigitalcollection.asme.org/heattransfer/article-abstract/102/3/489/441998/Experiments-on-Transient-Condensing-Flow-through-a

Any help or advice on this would be highly appreciated!

3 Comments
2023/05/19
14:16 UTC

3

Prandtl number at low pressure

Does anybody know any resources that might have the prandtl number of air at lower than atmospheric pressure? Or a credible source that shows that it's almost constant from 0,2 to 1 bar? I can't find any information on that even in any of my (German) literature... Unfortunately I can't access english Literature as easily, so any hints would help. Thanks

3 Comments
2023/05/18
12:07 UTC

0

Electrical Energy Consumption Balance

We know that E= Pt. Found this equation: dE/dt = E_supplied - E_measured(used) - E_losses. Trying to figure out how this came to be. Are there any references or literatures you can recommend?

2 Comments
2023/05/18
07:09 UTC

3

Special case in nusselt's number

So my professor asked us a question on nusselt's number, he asked in which case is the nusselt's number is exactly two. I thought it might be a condition where specific parameters in the formula are met but it isn't the case. Can anyone help?

3 Comments
2023/05/17
18:05 UTC

7

Can We discuss EOS?

Hi everyone,

I'm currently working on an application that will use multiple EOS's to solve for fluid properties for use in a larger application.

The trick is I need a GENERAL equation of state, the only inputs I have available (this is not changeable) are the critical properties, Vc, Zc, PC, Tc plus mole weight and acentric factor. That's it.

I've got PR and SRK working cleanly, but they have sub-par results for polar molecules.

I've been fervently searching for an EOS that would work better for molecules like water and etc.

I found a paper from 2015/2016 from Fluid Phase Equilibria by Forero and Velasquez titled "A Generalized cubic equation of state for polar and non-polar substances" that appears to have exactly what I need.

Here's the issue. I can't seem to get it to work. I don't know enough about EOS's to take the paper and calculate fugacity using it. The equations I've gotten to work so far had pretty straightforward equations already in literature.

I dont expect to be helped free of charge. If anyone wants to do some consulting or get involved please pm me. If anyone has another method or EOS they recommend, that would be great too.

There is one more I'm looking closely at from Valderrama in 2005 that appears to extend Patel Teja and create a generalized function that can be solved with Cardanos or the like.

PS. I tried chatgpt and all it did was lie to me. We have time before we're all out of jobs due to AI being infinitely better at math than us. Don't sweat it.

4 Comments
2023/05/12
16:00 UTC

6

Is there a difference in the chance for an atom to enter a pore with these two landscapes?

Both red and blue landscapes have energy barriers for an atom to enter the pore, but the blue landscape has an energy well between the barriers. Is the atom more likely to enter the pore in the blue landscape - i.e. does the energy well help offset the barrier? Is there a way of quantifying this effect? i.e. if the well was X kJ/mol deeper, how would this effect the likelihood of passing the barrier?

https://preview.redd.it/f5znr6ul98za1.png?width=348&format=png&auto=webp&v=enabled&s=91707499cdb507d3b022e7b572ef38486ba9bb1b

Note this isn't homework, I'm trying to understand some real world data.

Thanks.

7 Comments
2023/05/11
16:28 UTC

1

Thermodynamics of Steam Electrolyser

Hi,

I have a query regarding the thermodynamics of steam electrolysis.

Working from the reaction H2O -> H2 + 0.5O2 using standard enthalpies of formation, standard molar entropies and Hess’ Law, the total reaction energy demand ΔH is 248 kJ/mol at 800C whilst TΔS and ΔG have values of 62 and 186 kJ/mol respectively.

However, I have also calculated the value of ΔG using the expression ΔG = ΔGo + RTlnQ, where Q is the reaction quotient calculated using partial pressures. When PH2O, PH2 and PO2 are set to 0.9 bar, 0.1 bar and 0.21 bar respectively, ΔG has a value of 202 kJ/mol.

I realise this second route is likely the more ‘accurate’ method for calculating ΔG as it will reflect the operating conditions of the electrolyser however what effect will have on ΔH and TΔS? I assume ΔH will remain constant and TΔS will shift until the sum of ΔG and TΔS achieve the value of ΔH? Is this correct?

Thank you for your help

2 Comments
2023/05/10
22:37 UTC

2

Why do refrigeration cycles require adiabats?

1 Comment
2023/05/09
13:32 UTC

2

What are some good materials to go along with the “Thermodynamics for Dummies” book?

Currently I’m reading through the book and taking notes on it in order to get introductory knowledge into thermodynamics. If anyone has used the book or has any knowledge on it, what accompanying resources (stuff like quizzes to review and whatnot) did you use / would work well with it? Thank you!

12 Comments
2023/05/09
01:50 UTC

4

Is there a way to calculate the heat transfer coefficient in forced convection for Re>10^7?

I'm trying to approximate the heat transfer coefficient of a airplane wing and hull in different mission points and have the problem that the method for calculating the heat transfer coefficient i was taught in uni uses formulas for the Nußelt number that apply only for Reynolds numbers below 10^7 Is there a way to calculate the heat transfer coefficient that works even in these conditions?

4 Comments
2023/05/06
23:10 UTC

4

What are the diffrences between coolprop and refprop ?

I've only used coolprop in python. Is refprop developed by different team ? Is there python wrapper for refprop like coolprop ?

1 Comment
2023/05/04
17:41 UTC

6

How does pressure vary in the expansion section of a rocket nozzle ?

Hello, I am a bit confused on how the differences between total, dynamic, static pressures.

In a rocket nozzle's expansion section, the flow's velocity increases as it expands. I know this decreases "pressure", but which "pressure" is this?

I was under the impression that dynamic pressure is directly related to velocity, so it was my belief that dynamic pressure is not the "pressure" that decreases. Am I correct?

This isn't my homework, I've graduated, but it came up during a conversation and I've asked multiple people and have received no good answers .

When people refer to "pressure", which one are they typically referring to ?

1 Comment
2023/05/04
15:37 UTC

0

Water (liquid): P1 = 20 800 [kPa], T1 = 293.15 [K] & P2 = 19 000 [kPa]. T2 [K]?

Description

I am pressure testing a hose and am having a pressure drop of X bar. I "know" the initial Temperature (T1) to be ~ 20 [C] or 293.15 [K], after x hours of testing I had some pressure drop. I want to know if it is the temperature drop that caused the pressure to fall x bars, or if it is a leak!

Assumptions

Water is in a closed system (pressure testing a long hose with water).

Assume the hose to be infinitely ridged.

Assume a constant mass "no leakage".

Assume the phase state ( specific volume is v(f) [ kJ / kg K] ) of the Water to always be liquid.

Data

Volume of water is 60 [litres] = 60 [l] / 1000 [l] = 1 [m³], 60 [l] = 0.06 [m³]

Density of water at T1 ( T1 = 20 [C] = 293.15 [K] ) = 997.047 [kg / m³]

Mass of water is 0.06 [m³] * 997.047 [kg / m³] = 59.82282 [kg]

Type of pressure is Gauge / Hydrostatic / Any / easiest to calculate

P1 = 20 800 [kPa]

T1 = 293.15 [K]

P2 = 19 000 [kPa]

T2 = ? [K]

Solution so far

Can I use the following to find ΔT or T2:

Δu = C average * ΔT

↑ Source is book: Thermodynamics An Engineering Approach - 8th Edition

Chapter: 4 - Energy Analysis Of Closed Systems

Section: 4 - 4 - Internal Energy, Enthalpy, And Specific Heats Of Ideal Gases

Topic: Internal Energy Changes (page 184)

Excel sheet

https://cryptpad.piratenpartei.de/sheet/#/2/sheet/view/sI+KVwCbh+M3-iYFs2qyf72+TFvAnHRFYRPIIAhZB9c/

Click File, then Export to save the file locally!

Thank you for any hint or help :D

NB!

This is not a homework question, this something that I am dealing with at work. I just need the theoretical answer.

1 Comment
2023/05/04
08:05 UTC

7

Did you study engineering (undergraduate) in Germany?

If so, I have a question for you. I was curious about what a modern undergraduate Thermodynamics textbook used in a German university would look like. I am especially interested in looking at solved examples and the end-of-chapter homework problems.

So, I got “Thermodynamik Grundlagen und technische Anwendungen” by Baehr and Kabelac and was surprised by the fact that the entire book has only a handful of solved examples and no end-of-chapter problems or exercises. I also got “Thermodynamik: Ein Lehrbuch Für Ingenieure” by H. Windisch and noticed the same thing.

The typical American textbook (Cengel & Boles, Moran& Shapiro, Borgnakke, etc.) typically has almost 200 practice problems at the end if each chapter, so the instructor has a wide variety available to assign as homework assignments.

So I'm wondering if it's just not taught that way (lots of homework, lots of different problems) or is it like I've seen in Soviet era Russian books where one book is all theory and discussion and a “companion” book has only problems and exercises (and LOTS of them! old Russian calculus books are insane!)

Any information you have about this that you’d like to share?

8 Comments
2023/05/02
05:49 UTC

1

Does the specific enthalpy remains constant between states 2 and 3, and between states 4 and 1?

My professor assigned this problem and then decided to remove it because no one was solving it correctly. But now I’m still curious on how to solve it. I’ve tried 5 different solutions and none match up. Any advice on solving??

A turboprop airplane engine consists of three parts: adiabatic compressor, heater and adiabatic turbine. The compressor raises the pressure of ambient air (1 bar, 25C) to 20 bar. The heater raises the temperature of this air to 1500 K, holding pressure constant. This hot, high pressure air drives the turbine and produces work. Assuming that air is an ideal diatomic gas with constant heat capacity, answer the following for two cases: 1) the compressor and turbine are isentropic, 2) the compressor and turbine both have an isentropic efficiency of 80%. 1.For case 1 how much work is generated per mole of air passing through the system (kJ/mole, as positive number)

2.For case 1 what fraction of the heat provided by the burner is converted to work?

3.For case 1 what is the entropy change for the air as it passes through the system? (J/mole-K)

4.For case 2 how much work is generated per mole of air passing through the system? (kJ/mole, as positive number)

5.For case 2 what fraction of the heat provided by the burner is converted to work?

7 Comments
2023/05/01
01:05 UTC

13

James Webb's Incredible Sound-Cooled Camera | Primal Space

0 Comments
2023/04/29
14:44 UTC

1

Can someone explain why heat pumps can’t be used for an “infinite energy glitch”

I’ve just started the topic of heat pumps so please be forgiving about my lack of understanding.

Basically from what I’ve learnt, you can essentially take energy from an environment and transfer it to a “hotter” environment using some work input.

Let’s say I used a heat engine to transfer 20kj of energy to a well insulated basin of water. And to do this I used 5kj of input work.

Then say I used some relatively efficient turbine to transfer the 25kj of work inputted into my basin into 8kj of electrical energy.

Surely I could then take that 8kj of energy and use it to transfer another 22kj of heat energy to my hot water basin. This would then result in an even greater electrical energy gain.

My question is why can’t I just repeat these steps to turn my 5kj into essentially unlimited energy (so long as there is still energy in my cold source environment).

34 Comments
2023/04/28
19:31 UTC

1

Is there any way to convert ambient temperature to human body temperature and the other way around?

For example, let's say the ambient temperature is 19°C. What would be the same temperature for the human body ?

8 Comments
2023/04/28
12:30 UTC

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