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Questions tagged [thermodynamics]

Questions relating to the various laws of thermodynamics, transformation or movement of a form of energy to another form, primarily but not limited to heating or cooling of a fluid.

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Difficulty to reach Mach 1 by blowing into a small converging-diverging nozzle?

I'm learning about the basics of converging/diverging nozzles in my thermodynamics class, and it has me curious. Could you theoretically blow into a tiny converging/diverging nozzle and break the ...
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How to build multiple large scale fish tanks with water at different temperatures

I am wondering at a high level what are the material components that might be used in building several large scale fish tanks that are temperature regulated like on a fishing farm. For example, what ...
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Cooling Down Fridge Encased in wooden box

I'm currently working on a project which involves a small fridge encased in a wooden box. It's designed for hotels and the boxes vary slightly, but one common trait is that the box has an open vent ...
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How do you calculate the temperature after methane combustion if liquid H2O is sprayed into the combustor?

Calculating adiabatic flame temperature is pretty clear in thermo books, but how would you determine the temperature if liquid H20 was sprayed into the combustion chamber simultaneously? Thanks
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Solar Heat Gain/Outside Temperature Effects on a House

I'm currently a sophomore Mechanical engineering student and need some help with an equation for my project. I am coding a simulation in Matlab and have been given a very short time frame to do it. I ...
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38 views

Steam at 1 MPa, 300°C flows through a 30 cm diameter pipe with an average velocity of 10 m/s. The mass flow rate of this steam is:

Steam at 1 MPa, 300°C flows through a 30 cm diameter pipe with an average velocity of 10 m/s. The mass flow rate of this steam is: A) 0.731 kg/s B) 2.74 kg/s C) 3.18 kg/s D) 3.78 kg/s Can anyone ...
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Efficiency of a sterling engine

What is meant by thermodynamic efficiency of a sterling engine? and what are the factors that affect the thermodynamic efficiency of a sterling engine? Similair to other questions i know despite ...
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22 views

Doubt in proof of condition dG<=0

I tried proving the condition dG<=0 where G is Gibb's free energy and instead ended up proving dB<=0 where B is exergy. Can someone please explain what went wrong with my derivation. I have ...
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Is there a formula to relate the axial length of a nozzle expander with its radius?

I know the inlet, throat and outlet areas for a nozzle expander. Is there a formula to know its axial length? Something like L= f(A) = f(D).
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Gasturbine energy balance

I have a question regarding gasturbine energy balance, the total energy that flows into the gasturbine is given 12 MW and the power needed for the compressor is 4.4 MW and the turbine power is 9.3 MW....
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Is it feasible/possbile to do regenerative cooling of ScramJet or Ram jet powered aircraft surface?

Thinking of ScramJet/RamJet powered aircraft for unmanned vehicle. One of the challenges in designing such an aircraft is surface heating. Just like Rocket nozzles are cooled regeneratively with fuel(...
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29 views

What's the minimum area ratio required to achieve choking in Converging nozzle?

Is there any equation for minimum area ratio (Throat area (At)/ Combustion Chamber area (Ac) ) required for choked flow in convergent nozzle (just like equations for minimum Pressure ratio and ...
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Feasability of melting through ice crust with radiocative heater

I'm pretty interested in space exploration. Here I'm interested in a moon of Jupiter (Encealadus) which is covered by a thick ice crust, but apparently has at some depth a liquid water ocean which ...
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What is the meaning of dew point -20 or -40 or -70 deg.C? Since below zero centigrade, water cannot dew (already freeze)?

I saw these specifications listed for a compressed air dryer (desiccant type). I also wonder, can we extract moisture without make it liquid (dew) first when in it is in a vapor state? How would we do ...
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34 views

equivalent deduction of Non-mechanical thermodynamic work

"Work (as in Thermodynamics) is said to be done by a system if sole effect on the surroundings could be the raising of a weight." So, how is non mechanical work like : Chemical work, Magnetization ...
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Minimum distance between long curtains and radiator for good air flow?

It's a known fact that you cannot place a curtain near a radiator because it will trap hot air behind the curtain and also block cold air from going through the radiator from bellow. But obviously ...
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50 views

Calculating the Surface Temperature of a Cable with known heat flow

I have a cable which dissipates $\dot Q_{total}$ and I need to determine the surface temperature of the insulation $T_{ins}$ and the surface temperature of the conductor $T_{cond}$. There is an ...
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44 views

Fluid Mechanic Buoyancy problem. How to determine the height?

I tried to solve it but I don't know how to bring the Area A in this equation. I used the area of the small vessel, but this one shouldn't be in the equation. Thanks a lot
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Air conditioner using liquid nitrogen and a cryogenic heat exchanger

I have been trying to understand if compressor-based systems in air conditioners can be replaced to make something more economical and energy efficient. From what I understand, the compressors ...
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1answer
35 views

calculating the efficiency of a microwave oven

If I want to calculate the efficiency of a microwave oven, I have the input Energy (= power (watts) x time (s)) and the output which I measured using temperature difference (q=mct). So do I divide ...
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Enthalpy vs Heat

If we have a closed cylinder filled with a gas and we heat it, by $1^{st}$ law we have: $\Delta q=\Delta U$ (?) since $\Delta V$ = 0. But $\Delta H= \Delta U+ V. \Delta P$. What does that difference ...
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33 views

Industrial scale heat pumps

I've read (1) that the efficiency (COP) of a heat pump does not change much with size. If that is true, why don't power companies employ them to produce yet more power from fossil fuels or renewables? ...
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How to calculate adiabatic air temperature loss caused by increase of humidity?

I'm searching for an approach on how to calculate the adiabatic temperature loss of air when increasing its humidity by a humidifier. Assumed we have the following setup: Air measuring position 1 (...
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63 views

Practical Application of Adiabatic processes

The concept of Adiabatic is basically the isolation of a system in a realm where energy doesn't flow either way. Adiabatic compression and expansion is a common phenomenon harnessed in equipments and ...
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Designing a submersible cooling system for rapid flow hot nitrogen gas [duplicate]

I'm trying to design a cooling system for cooling nitrogen at 300°C. My original design (which is very open to modifications) is an aluminum ring block with exterior fins and vertical holes through ...
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Is thermodynamic data of fresh water a good approximation for saline water?

I am currently working on desalination project for my final year. The design is based on flashing of hot liquid in low pressure vessel and then condensing the extracted the steam to produce pure water....
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Would an afterburner on a turbine drastically improve acceleration lag?

A little bit of context. My friends and I were watching the "Dark Knight Rises" and a discussion about the Batmobiles ensued. I, being a former ME student, said it was stupid that batman has a turbine ...
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166 views

Cooling Hot Nitrogen with an aluminum block

I'm designing a cooling system to cool nitrogen gas stream at 300°C with a mass flow rate of 15.6 g/s moving through an undulating channel in a cylindrical aluminum ring block with external fins. How ...
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59 views

How can I calculate the amount of heat my copper water trap can dissipate?

My small (1.5hp) air compressor is connected to a water trap made of 8mm copper of about 4 metres length. The idea is that you push the compressed air through the piping which cools it, condensing the ...
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2answers
47 views

Does a tank of liquid nitrogen increase in total internal energy after emptying?

Suppose there's a 10 liter perfectly-insulated sealed tank of saturated liquid nitrogen at 4 bar. Therefore the fluid has properties, $\rho=740 \,\frac{\text{kg}}{\text{m}^3}$, $m=7.4 \,\text{kg}$, $\...
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Why can we extend the derivation of delta(S mix) to ideal solution from ideal gas behavior?

The formula for change in enthalpy for ideal gas is derived using ideal-gas law which cannot be extended to ideal solutions (my intuition). Then why can we extend the formula of delta S mix to ideal ...
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39 views

Classic Thermodynamics: Ideal gas question

The question is in the attached picture. I know how to solve the first part of the problem, but the second part confused me a lot. Now when the oven (system) is unsealed, doesn’t make that an open ...
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How to derive phase equillibrium criteria for more than 2 phases mathematically?

I know the derivation of phase equilibrium criteria derivation for 2 phases. I am not getting how to extend the derivation to more than 2 phases. Please help me out on that. Thank you
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Evaporating temperature of NH3

Considering a flooded evaporator (using plate heat exchanger - PHE). Cold NH3 liquid from a surge drum supplies to the PHE to cool down water from 30 oC to 3 oC. The evaporating temperature of NH3 is -...
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Deriving Enthalpy in terms of (T,V) and (T,P)

We are being asked to develop thermodynamic expressions for the enthalpy of a system in terms of (T,V) and (T,P). I understand that the Legendre Transform can be used to find H=H(S,P), but I'm getting ...
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Developing a model of rate of cooling of steel tube in air

I am trying to develop an equation to determine the cooling rate of a steel tube in air. I'm using Fourier's Law, Stefan-Boltzman Law, Newton's Law as well as the specific heat capacity equation. The ...
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49 views

Fit NRTL parameters for an immiscible system

Problem description: I need to know the liquid-liquid eqilibrium behavior of a THF-water-toluene system. I need to use NRTL because UNIFAC does not predict THF-water systems accurately. I have ...
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30 views

Calculating geothermal heat pump

So I need to calculate how big geothermal heat pumps I need for a building. Heat pump is used for heating and to heat used water. If I know how many kW's are needed for heating the building and the ...
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101 views

Save money with using a refrigerator correctly - blowing cold air to the back of it?

We are heating up our houses in winter to ex.: 21C to then have an insulated box inside the house, which we cold down to 5 and -15C, and we call it a refrigerator.. :) The situation: we would buy a ...
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40 views

Working of a fan or propeller

I can roughly understand the working of an axial flow gas turbine i.e. the fluid moves over the blade and gets redirected, its whirl velocity component changes and the blade is pushed in the opposite ...
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38 views

Is using a radiator to warm water inefficient and if so why

I'm currently designing a Hottub for my scouting. This isn't the first time we are doing this. In the past, we build a wood stove around a radiator as shown above. Now I'm wondering if this is ...
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Why boiling occurs when vapour pressure equals the local atmospheric pressure

When we increase the temperature of water in an open container, the vapour pressure increases.But boiling (phase transition at constant temperature and pressure) occurs only when the vapour pressure ...
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46 views

Is this refrigeration cycle thermodynamically possible?

This is a screenshot of a lab where we were experimenting Vapor-Compression Refrig. Systems. I am trying to understand what is happening between points 1-2 because I thought that because of max ...
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Looking for something to reflect ultraviolet light at extremely low wavelengths

I have been working on a product that involves the reflection of low wavelengths of UV light (200-250nm). However, I have not come across many materials that are able to do so and the high-...
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2answers
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which type of casting will give higher cooling rate, casting in thin water-cooled copper mold or casting in thick copper die?

It is will known that increasing cooling of a metal casting will lead to finer micro-structure which mean higher mechanical properties. If I have 2 molds one is thick copper die ,& second is just ...
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1answer
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Connect heated copper tube in PVC tube

I have a copper tube that I'm heating (quite a lot) with a torch, I have to connect a PVC pipe in the entrance of the copper and one in the exit (so water can flow through the copper pipe). The only ...
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Engine power and torque of a vehicle

Having the Power/Torque-RPM curves of a vehicle, how can we derive the engine power equation and the torque out of these curves with respect to time?
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22 views

Derivation of specific helmholtz function relationship for an ideal gas

The question is to prove the following form of helmholtz function for an ideal gas $$f=C_v (T-T_0)-C_vT \ln \frac{T}{T_0} -RT \ln \frac{V}{V_0} -s_0(T-T_0) +f_0$$ I tried using $$df=du-Tds-sdT$$ Now ...
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1answer
52 views

Wet bulb temperature in psychrometry

"In a sling psychrometer(with dry and wet bulbs), if the moist air is not saturated(with water vapour) then it evaporates some water on the wet bulb and in turn cools the wet bulb.So the wet bulb ...
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1answer
40 views

Conductive Heat Transfer calculation over time

I am trying to heat the oil in a 1/2 inch thick rectangular steel box by using an Insulated heated blanket covered on all 6 faces of the box, the working conditions of the operation are at -50 Celsius,...