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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Usage of Thermodynamic Steam Tables

Having both the pressure and temperature for saturated-water in a particular state of a cycle, I was interested in its enthalpy. When looking at steam tables I am not sure if I should use the pressure ...
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How to calculate the volume of air that will cool steel?

The task is to cool steel pieces in boxes with indoor air using a fan. The number of boxes is 6 pcs, each weighs about 15 kg. It is required to cool them from 90°C to 30°C with using 30°C air in 9 min....
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In case of solvation , Who converts gaseous ions to aqueous ? Why do we even have gaseous but not solid ions?

We take Nacl and dissolve it in water. There is formation of $Na^+$ and $Cl^-$ gaseous ions along with 2H+ and $Cl^{2-}$ aqueous ions. Then , Na+ is attracted towards $Cl^{2-}$ and 2H+ is attracted ...
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Cold Plate Thermal Resistance

I am designing a cold plate that will have 4 Power Modules Mounted on it. The total power loss from the modules is 5000W(1250W per Power Module). I have simulated the results on ANSYS Fluent and ...
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Do all thermoelectric generators (TEG) require two distinct metals?

Most thermocouples that I've seen being sold online consist of two separate metal wires connected at junctions. Many smaller thermoelectric generators consist of a combination of iron and copper or ...
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Mathematical derivation for 1st law of thermodynamics

Mathematical derivation for 1st law of thermodynamics : What I know : 1st law of thermodynamics is known as or derived as the conservation of energy . But how are the terms U , q and W derived from ...
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43 views

What does it mean by average kinetic energy of molecules as temperature?

We say kinetic of molecules as a function of temperature from the equipartition theorem. This means that more is the velocity or mass of the body , more the is the temperature. My Q is why does ...
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Difficulty in understand the meaning of standard enthalpy of formation

Standard enthalpy of formation is when one mole of compound is formed from its constituent elements. Example : $H_2$(g) + $O_2$(g) —>$ H_2O$(L). But we don’t say the same for $CaO + CO_2$ —> $...
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46 views

surface temperature due to constant heat flux

Imagine a hollow cylinder with the outside surface exposed to air (convection). If there is a constant heat flux on the inside surface how would I calculate both the inside and outside surface ...
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What is a safe and efficient temperature range for absorption chiller generators?

I've read that the role generators play in the absorption chiller cycle is to concentrate the salt solution so that the evaporator/absorber can maintain a vacuum by constant absorption of vapour ...
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Can I consider this scenario for the phase change of $H_2O_{(s)}$ to $H_2O_{(L)}$

From a particular topic : Enthalpy changes during phase transformations. It says in my textbook that standard state of a substance at a specified temperature is it’s pure form at 1 bar pressure. Let’s ...
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Why ΔH has two separate equation in my textbook?

ΔQ = ΔU + Δ(PV) (First law of thermodynamics) which has the same statement as the 1st equation. $Q_p$ or ΔH= ΔU + PΔV which is the heat exchanged at constant pressure. Then why in the image above in ...
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Difference between temperature and heat

It is becoming very difficult for me to understand the difference between temperature and heat. About heat all I have read : Heat exchange is related to as exchange of matter + energy. Heat is ...
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What is heat , chemical energy and thermodynamics laws restrictions

Below is a text over which I have my questions which I have written below this text: Chemical energy stored by molecules can be released as heat during chemical reactions when a fuel like methane, ...
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Why does temperature not increase by putting more logs into the fire

Why does temperature not increase by putting more logs into the fire ? It is a very common observation which I have seen but after some research online I found that it is not completely true. One ...
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Difficulty in understanding the scenario for this Q

Q:If a closed rigid wall container filled with 20 gm of He moving at a constant speed of 10 m/sec is stopped abruptly to stand still, find the increment in temperature of this He sample. My question ...
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Internally reversible and externally reversible processes

I am now reading about how to calculate entropy changes of a system undergoing a process, and the textbook says that to be able to calculate such a change, you need to imagine that the system is ...
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39 views

Do throttling devices need valves?

Why is it that a throttling device always has a valve leading to it? If in a closed system loop, a cooled gas exists right before the throttling device, shouldn't the sudden decrease in cross-...
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94 views

How does steam in almost vacuum condense at 35 Celsius?

I read about absorption chillers and found out that the water is evaporated/boiled off at the generator. Water boils over at about 3.7 Celsius in the near-vacuum conditions in the generator. Now the ...
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38 views

Absorption chillers: can they achieve temperatures below zero Celsius?

If I understand correctly (history shows I probably don't), then the only reason why the absorption chiller evaporators are restricted (I hope that's the right word) to extract just enough heat so the ...
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Solidworks flow simulation temperature drop

Using Soldiworks flow simulation, I want to calculate the temperature drop of water (40 degrees) which flows in 10 m steel pipe. How should I manage Wizards setting and goal?
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What is an advisable temperature for drying sand in an oven, for later use as an epoxy filler?

I want to dry some sand before I use it as an epoxy filler (making a machine base). I would like the sand to be drier than when I started (so that I am not adding >=1% water that has condensed in ...
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Thermodynamics - Property Tables

I had a quick question regarding a homework problem. I cannot seem to understand why we are using internal energy of evaporation for this problem, instead of just using the internal energy of vapour ...
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Response time and time constant of Mercury thermometer

I searched to find differences between response time and time constant of a thermometer. So I found that those two are not same and we have to get 99.3% of the total step change to find response time ...
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Why is potential energy = 0 for ideal gases and not for non ideal gases?

Why is potential energy = 0 for ideal gases and not for non ideal gases ? The reason is in my textbook that average between the atoms increases or decreases due to bonds being broken.So , my Q is that ...
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Is heat transfer = potential energy in formula of 1st law of thermodynamics?

According to my textbook , 1st law of thermodynamics is Change in internal energy of ideal gas = K.E + P.E = Work done = $F_{ext}$ * displacement$_{walls}$ = n * degree of freedom * R * $\delta$T / 2 +...
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Thermodynamics Adiabatic process

This is the question please: 1 kg of steam is at 100 bar and 375 °C expands reversible in a perfectly thermally insulated cylinder behind a piston until the pressure is 38 bar and then steam is dry ...
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Specific CO2-Emissions of Natural Gas & Hydrogen Mix [closed]

consider I have a mixture of Hydrogen and Natural Gas. The volumetric mixture is $\varphi_{H_2} = 70 \%$ and $\varphi_{NG} = 30 \%$. So the $H_2$ makes up a mass fraction of $$w_{H_2} = \frac{\rho_{...
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Does electrolysis of warm water differ from electrolysis of room temperature water?

Typical energy efficiency of water electrolysis is about 75%. Will this change with the temperature of water?
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70 views

How much water do steam turbines consume per kWh of electricity generated?

I know this depends on some factors, and I can only think of these: the inlet steam's temperature and pressure. If there are more factors then please do tell me. What is the maximum figure in liters ...
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35 views

Is it possible to avoid the emission of CO2 and other greenhouse gases from thermoelectric plants?

Since the deleterious effects of greenhouse gases are well known, I was wondering if it is possible, for example, for thermoelectric plants to avoid this kind of emissions. More exact, I think it ...
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67 views

Does adding cold water to evaporative air coolers actually produce colder air?

Or should it be the other way around, because hotter water evaporates easier than cold water? I know this sounds stupid, so just hear me out. I use an evaporative cooler and add normal room ...
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How can the Saturated Liquid Line be two phases?

TLDR Why do all of the resources I find say that any point along the saturated liquid line is 2 phases? How can statement 1 and 2 both be correct? Steam Quality @ p,v,t on sat liquid line= 0 =>(...
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Will pressurizing container slow moisture permeation?

I have some electronics housed inside a sealed plastic enclosure, which will be used in a very humid environment. The environment can be assumed to be 100% humidity at a constant temperature, all the ...
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PV Diagram Pressure temperature volume increasing what is net effect on output?

I am not properly understanding this question. please help! Most of the process on PV diagram have this kind of curvature But in the question above, its bit weird. My thoughts: Pressure, temperature ...
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Calculating final temperature of humid air and cool stone

I'm trying to gauge the efficiency of a night-cooled stone to condense water from the humid air at seashores at noon. I'm assuming the stone is thermally insulated once it's in thermal equilibrium ...
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What is basic knowledge wanted from a bachelor mechanical engineering student before joining? [closed]

At the moment, I just finished my senior school year and in a few months, I will be studying mechanical engineering as bachelor degree. However, I don’t like to waste my time and want to use this time ...
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Find the irrigation pumping rate (mass flow rate of a pump) versus time

Here is what we are given. Time ranges from 0 to 10 hours, with t=5 being solar noon. $\dot Q = 90Asin(\pi t/10)$ Watts, where A is the solar collector area in square meters and t is the time after ...
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Can Peltier Modules be used as an Air Conditioner? [duplicate]

Air Conditioners are very expensive so I'm planning to build a split type air conditioner using Peltier modules for my sealed room that is 6.5 length, 7 feet height, and 5.6 wide. I plan to use nine ...
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Thermodynamics problem for school

I will try to translate it as best as I can T = temperature Pi = P initial Vf = V final Monoatomic gas The T doubles isobarically (process 1-2), absorbing heat Q12=250 J. The gas is then cooled ...
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What is the energy and capital cost of NH3 from Haber Bosch plant that takes in pure H2 and N2?

A question that becomes particularly relevant for "the hydrogen economy" or in the event that IGCC coal baseload electric generation seeks off-peak markets for hydrogen. I can't seem to ...
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HVAC Condensor Liquid Level/Ratio Estimate

Let's say I have a simple condenser inside of a HVAC system with 1 point of entry(Super-heated Vapour) and 1 point of exit(Saturated Liquid) for my refrigerant r134a. A pipe passes through it with ...
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Inconsistencies with units for thermodynamic identities

I am working out the temperature for an Ideal diesel cycle: Based on the isentropic properties for an ideal gas. $$T_f = T_i * r^{k-1}$$ Where $T_i = 50 ºF = 510ºR$ If I use ºF, then $T_f = 172.9ºF = ...
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Specific heat at constant volume used in non constant volume processes

I have commonly seen that the work for an isentropic compression is written as: $$ W_{in} = c_{v}(T_1 - T_2) $$ What confuses me is that since it is a compression (volume is changing) why do we use ...
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Temperature change during phase change of water

Suppose we have a mass of water at room pressure and temperature, then we started heating the water. What will happen to the temperature if we decreased the pressure while the sample of water is ...
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Is cast iron better that Copper or Aluminum for cooking steaks?

Folk wisdom states that Cast Iron pan is the best for searing steak because of its high heat retention relative to the aluminum/copper counterpart. However, this doesn't make sense in practice... ...
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When can the thermal and mechanical energies of a fluid control volume be considered separately?

For a fixed control volume of a fluid, the conservation of energy equation is typically written as something like $$\dot{Q} - \dot{W} = \frac{\text{d}}{\text{d}t}\iiint_{CV} \left( \hat{u} + \frac12 V^...
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Can't tesla turbine work on air heated by solar concentration? [closed]

If we heat air along a pipe that's tilted at 30° to the horizontal using parabolic troughs, the air should rise upwards (I'm not sure how fast or powerfully) and then connect the outlet at the top ...
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Is 'stacking' engines a viable way to maximize efficiency?

Suppose we have $n$ carnot engines connected together labelled as $C_1,C_2...C_n$, now suppose we feed an energy of $E_1$ into $C_1$, now by the second law of thermodynamics, it must be that ...
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Is high pressure combustion hotter than atmospheric pressure combustion

let's say i have the same quantity of coal and air. Will the combustion achieve higher temperature if the air is under pressure?

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