/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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    We welcome 'wordy homework questions' and discussion on relevant topics. If you have input values to a problem and need help getting to the output values, give us some context! Let us know what you've already tried, what you're struggling with, and be prepared to engage with follow-up discussion. Be prepared to do the final calculations yourself. We wont do your homework for you. No copy+paste lazy cheaters.


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12,274 Subscribers

1

prerequisite chem for Mech Eng Thermo

Next semester I was planning to take mechanical engineering thermodynamics. Now I am reading Brown's Chemistry: The Central Science, what sections of the books should I just read for the preparation on learning Mechanical Engineering Thermodynamics? Thanks in advance!

https://preview.redd.it/mr5y2fjtz2pc1.png?width=584&format=png&auto=webp&s=c812be7230a622c35f9b15bd2c553be7c99da856

4 Comments
2024/03/18
11:51 UTC

7

If heat is the lowest form of energy, then how does geothermal electricity generation work without violating entropy laws

Please explain to me like I am 5

21 Comments
2024/03/17
19:42 UTC

2

The Most Misunderstood Concept in Physics

Nice intro to the concept of entropy.

6 Comments
2024/03/17
13:13 UTC

2

How to calculate specific heat capacity

I'm wondering if it's possible to calculate specific heat capacity of any molucals like CO2 or N2 in under P=constant condition at any given temperature

4 Comments
2024/03/17
09:50 UTC

1

I'm taking Thermo2 after a year of Thermo1 and I forgot a lot. Whats a good quick review method you suggest?

Like should I just review what a Carnot cycle is and solve a couple of question or go take a YouTube course or what? The latter is quite time demanding and I would like if you have a summarizing content somewhere online or a crash course that is short or something of the sort.

5 Comments
2024/03/13
22:23 UTC

1

Hollow cylinder heat transfer

Hello, maybe someone here can help me. I am trying to solve the following problem:

A hollow cylinder with outer diameter r_a, inner diameter r_i and a layer thickness s is heated. I measure the surface temperature of the outer wall (t_a), and look for the temperature that occurs inside the cylinder (t_i).

Values: r_a= 51mm , r_i=46mm, s=5mm Lambda =15 W/(mK) (alloy steel) t_a = 50°C t_i= unknown

Can anyone help me? I'm a bit stumped...

1 Comment
2024/03/13
19:34 UTC

0

Nusselt correlation flow between two cylinders

Hi everyone,

Currently busy with a heat transfer problem in which I need a Nusselt correlation (not sure if Gnielinksi relation can be used). In the situation the fluid flows between two cylinders as can be seen in the sketch. Does anyone have tips for any appropriate Nusselt relation? Fluid is operating in the turbulence region.

4 Comments
2024/03/12
12:50 UTC

2

Will this oil heater design flow by convection?

Hi thermo -

I'm building a greenhouse and am looking to build a low-intensity thermal mass heater. While there are plenty of options I could go with, one of the most accessible in my situation is a combination of a thermal mass heater with radiant oil heating.

My primary question is whether a single heating point will be enough to have heat transfer flow within the system?

The idea (image attached) is to store a decent volume of water and/or cooking oil inside of black steel pipe (oil because of thermal mass and lower boiling point of water), introduce a 12v or 24v heating element into a reservoir, and power it with solar panels with a battery backup system. Would cost me a few hundred bucks and, if it works as I intend, will warm up my soil beds so they can also provide a bit of thermal mass. There are probably better ways to go from scratch (geothermal, for starters), but this is the most accessible I have at this stage.

Will a single heating element create sufficient convection to have warmer, lighter oil flow within the pipeline network? Or is a pump a must in this situation?

Thanks in advance!

https://preview.redd.it/07iuaijj7snc1.png?width=1513&format=png&auto=webp&s=4ab8702d7c41773b9960ef05d59bec96176aaae4

8 Comments
2024/03/11
22:32 UTC

1

how often is 3 dimentional heat transfer equation really used in industry

Currently learning 1 dimensional heat flux equation, and gets overwhelmed after i saw the 3 dimentional ones, and i was wondering do i really need to learn and understand this equation to proceed my career into CAE? also how often is it to be used in CAE?

4 Comments
2024/03/11
18:25 UTC

1

Is the steam from a kettle liquid droplets condensed in air or gaseous water? What does the distribution of energy look like of that steam when divided between kinetic and heat?

4 Comments
2024/03/10
15:33 UTC

2

Fundamental property relation

Obtaining the equation of dH = Tds + VdP can be done by differentiating H = U + PV and substituting the appropriate equations. However, why do we have to multiply the expression by n (moles) afterwards?

3 Comments
2024/03/10
01:17 UTC

1

Closed system: heat, air and water

I have water and air in a closed system at rtp. This closed system has a relief valve (set pressure is tbd). If the water is heated, through a heater/heat exchanger, for the purposes of boiling, what is the pressure of the closed system when the water boils? I’m assuming the water will boil at 100C? Im trying to decide on the set pressure of the PSV.

1 Comment
2024/03/09
20:31 UTC

2

Simple thermodynamic problem I'm unable to solve

I need help solving this simple problem and somehow I'm having a really hard time.

Imaging a 36ft^3 enclosure with a 1500W heat source inside. The enclosure is filled with nitrogen and has a nitrogen input port and an exhaust port.

I want to calculate the required nitrogen gas flow required to maintain the ambiant temperature of the enclosure around room temp (by replacing the "warm" nitrogen atmosphere by fresh ambiant temperature nitrogen. Let's suppose the nitrogen input is 20C, and the maximum enclosure temperature 30C.

What formula could I use?

Thanks!

2 Comments
2024/03/08
17:51 UTC

1

Thermodynamics Research Paper

I’ve been tasked with writing a 1-2 page research paper analyzing any thermodynamics article I choose. I just need to summarize an article and then critique it.

I’m looking for any suggestions on an interesting topic, anything cutting-edge or aerospace related would be interesting to write about.

If anyone has seen any interesting articles recently it would be much appreciated if shared!

2 Comments
2024/03/08
06:35 UTC

1

Maximum work done

A well insulated 5 m3 gas cylinder contains air initially at 1.5 MPa and 450 K. Calculate the maximum work that can be obtained by opening the cylinder valve. Assume air to be an ideal gas with a constant specific heat capacity of Cp= 50 J/mol∙K. Surroundings temperature and pressure are 298 K and 0.1 MPa, respectively.

Anyone knows how can I get a maximum work here? Is an adiabatic process.

2 Comments
2024/03/07
21:08 UTC

1

Closed box cooling

Greetings,

Im going to make a box which will contain Battery (jackery 500) and WiFi Acess point inside. This box will be outdoor in rainy and very sunny enviroment aswell. Problem is cooling. My current plan is to cut hole for peltier module, one big CPU cooler on outside (hot size) and smaller one on inside side with smaller heatsink and pc fan blowing into battery. Of course some kind termostat is needed to not going into negative celsius temperature and not creating ice.

I choosed this plan so i dont cut holes for air intake, because im afraid of moisure, because in my country there is hight moisure at the morning.

I dont know much about thermodynamics, so thats why im here. My question is, is my approach good ? Do I even need peltier module, or high RPM fans inside closed box (without fresh air intake / "outtake" for heated air will be sufficient ? How to solve this so i can use my box outdoor ?

Thanks in advance.

1 Comment
2024/03/07
09:09 UTC

5

Is Pure Water Always Saturated

In the rankine power cycle, it is given the pump converts low pressure water to high pressure water. Since it's pure water(and pressure is given) is it safe to assume it's saturated water and thus system is specified and use the saturated steam pressure table for pure liquid to obtain the other thermodynamic variables?

5 Comments
2024/03/07
00:57 UTC

2

How can i find pΔV??

hi guysss im taking physical chemistry and need help with this problem I've tried using the perfect gas formula but i know its wrong idk what to do desperation is on the prowl the question goes: the reaction C2H5OH(l) + 3O2(g) -> 2 CO2(g) + 3H2O(g) ΔrU=-1373 kJ mol-1 298 K and 1 atm. calculate the ΔrHº

I already know it has to do with the formula ΔrHº = ΔU + pΔV

any help would be appreciated!! thank you

2 Comments
2024/03/07
00:39 UTC

1

Specific enthalpy of wet steam without the dryness fraction

how would you find the specific enthalpy of wet steam at a given temperature if you have not been given the dryness fraction? would you do hg-hf from the steam tables?

1 Comment
2024/03/06
18:56 UTC

0

Hey so Im taking college level thermodynamics next semester. I kind of cheated throughout my chemistry and Physics 1 course. I can prob review and do general problems. Am I fucked when I take thermodynamics?

17 Comments
2024/03/06
04:08 UTC

2

Flaw in my logic?

Imagine cooling a house with water. Here's my thought process:

The mass of air is equivalent to a volume of water 1/800 in size(owing to their densities, and as a rounded number). The specific heat of air is around 1/4 of water. Therefore, in order to achieve a 1 to 1 temperature change, I can use 1/3200 the volume of water as I have air in the house. For a 1700 Sq.Ft. home, and assuming 8ft heigh walls, I come to needing approximately 4.25 Cu.Ft. of water, or about 32 gallons.

If the water can be contained (say, in a plastic barrel) 5-10 feet below ground, I could pump the reservoir to a heat exchanger, thus supplying a heat pump with a near constant temperature. (I will need to know how quickly the reservoir will bleed off its heat into damp sand, and there may be the flaw I'm looking for)

I am assuming the house air will raise about 5% of the temperature difference (from inside the house to outside temperatures) per hour. I also assume the daily average outside temperature, taken during the hottest month, will be around 95F for each of the 24hrs in a day. I want to keep the inside temperature at 65F. Hence, the inside air temp will raise by 1.5F each hour, and will be cooled by that amount each hour due to the heat pump. Thus, the water reservoir will increase by 1.5F each hour, and subsequently, with no heat transfer from the water to the ground (which we know there will be some), will achieve a temperature of +36F over stable ground temperatures (probably around 70F in summer with no heat load). Doubling the volume of water should halve the temperature increase, which seems somewhat necessary to be efficient.

So I suspect the linchpin in my idea is the rate of heat transfer between the reservoir and the earth, but I'm not sure how fast energy is transfered through infinite earth.

Any comments are appreciated.

9 Comments
2024/03/05
19:32 UTC

5

Hi, does some body know what the SdT is in the Helmholtz free energy equation

I know that TdS is the amount of heat absorbed by the system but what is SdT?

2 Comments
2024/03/04
18:54 UTC

2

Is there a good youtube channel or something that teaches around 200 level thermodynamics and that actually makes it interesting?

It doesn't seem like asking this is against the rules, but if it is, please let me know.

0 Comments
2024/03/04
04:25 UTC

2

When can you approximate a compressed liquid as a saturated liquid?

In lecture our professor never went over this concept, and when asking my friends with a different professor for help on homework, they told me you can approximate a compressed liquid as a saturated liquid and use the values from the temperature table at the same temperature. For this instance, the water is 10kPa and 40 C, but saturation pressure at 40 C is about 7.4 kPa, which seems like too large of an approximation to me to make. Is there like a cutoff of when you can approximate this? I'm just used to all the classes I've taken up to this point requiring exactly specific values, and making approximations is something I find hard to judge when to do it.

3 Comments
2024/03/03
18:50 UTC

4

What are some examples of things we colloquially think of as “heat,” but really aren’t?

I don’t know if this is too philosophical, but I still thought this would be the best place to ask. I have been reading Lavoisier, who seems to see “heat” as a sort of measurement or a phenomenon of feeling. To him it’s a reaction to the “cause of heat.” I am curious if people here think this aligns or differs from common perceptions of heat. Do we think of heat as a reaction or do people see heat as something in itself?

5 Comments
2024/03/03
17:50 UTC

2

Moving heat from air to water

Am I correct in thinking this way? I'll be taking general numbers here and am focused on nailing down the concept rather than very specific numbers. I'll use specific numbers when/if it matters.

Water is about 800 times dense as air. It also has about 3 times the energy storage owing to the respective specific heats. So if I want to transfer energy from a given volume of air such that the temperature loss of the air is the temperature gain of the water, I could use about 1/2400 of that volume of water?

Again, this is ignoring efficiency and is taking generalized numbers.

2 Comments
2024/03/02
21:57 UTC

4

How to solve Delta H?

Im not understanding this part

deltaH[total] = n(deltaH[melt] +Cpm • DeltaT + deltaH [evap])

I understand the part on DeltaH = n • Cpm • DeltaT, but why do we have multiply n to the other Delta Hs

should I assume that the DeltaH they gave is in terms of (Cpm • DeltaT) or (Cp•DeltaT)

Please enlighten me. Thank you!

5 Comments
2024/03/01
14:09 UTC

3

Why is the answer -5.28kJ?

Everywhere on the internet in the process of the answer, the mass required to be put in the eqn q=Csmdelta T is 125g ie mass of water only when it should be 145g (total mass of the system) as in the reaction both water and nh4no3 play a role. Why is it taken only 125g?

6 Comments
2024/03/01
08:06 UTC

3

Closed System Pressure Gradient?

Im hoping someone may be able to shed some light on this problem as my thermo knowledge is not what it used to be. We have a closed system with none/extremely minor changes in volume. The temperature is increased at location (1) and location (2) remains at ambient temperature. We have a couple theories with what happens to the pressure. My vote is that it raises in (1) and eventually equals out over the entire system. But some of my colleagues believe there may be a pressure gradient along the length of the system. The length of the capillary is over 5m

5 Comments
2024/02/29
10:24 UTC

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