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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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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Does a photon impart energy on a surface (to raise its temp) if it goes THROUGH that surface?

Hi so I have this senior design project involving cooling human skin as it is irradiated by an infrared LED. Human skin not only is super absorbent to IR, but that IR goes THROUGH the thickness of it ...
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Why does my heat pump condenser outlet make a single pass through the evaporator before the expansion valve?

I'm hoping to modify a hot water heat pump to capture both the hot and cold side. I'd work with an HVAC shop to make the modification but want to understand the circuit. It's not the simplest of units ...
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Thermoelectric cooler parasitics

So I'm looking at performance curves for thermoelectric coolers, and it seems like these curves are so misleading as to be useless unless I'm missing something fundamental. Suppose I have a ...
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Optimizing flue gas temp for maple syrup evaporator

I have a simple maple syrup evaporator. Fuel is burned in a firebox located under 1mm stainless steel pans containing maple sap--which becomes syrup once enough water is evaporated. After passing ...
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1answer
36 views

Why do binary cycle geothermal power plants have so low efficiency?

Well, a binary cycle geothermal power plant utilizes the low temperature of the primary liquid to generate heat. I found out that they are only 10% to 15% efficient. Is the continuous flow of the ...
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1answer
37 views

Heat rate through composite wall with non-constant thermal conductivity

I just started a job working with cryogenics and I am trying to solve an interesting heat conduction problem that I was hoping someone could help me along with. I have a structure that spans from room ...
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1answer
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Assumption of specific enthalpy

Given an adiabat turbine with state 1 fluid data as: $$h_1 = a$$ $$p_1 = b$$ $$t_1 = c$$ and state 2 data fluid data as: $$h_2 = unknown$$ $$p_2 = e << b$$ $$t_2 = f < c$$ $$h_2' = g$$ $$h_2''...
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Why coasting is not used to save fuel?

Why a technology like freewheeling not used in cars to save fuel? We all know that when we move to neutral gear (coasting), the car travels farther than when it is in gear because there is no engine ...
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finding a volume after non-isentropic expansion (constant heat approach)

Nitrogen (N2) in a piston cylinder device undergoes an expansion process from State 1, T1=500 K, V1=1.0 m3 to State 2, T2=700 K. If the specific entropy change (s2-s1) is 0.468 kJ/kgK for the process ...
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How to calculate the ideal and realistic estimate of thermal efficiency of absorption chiller?

How Can I calculate the ideal and a realistic estimate of the thermal efficiency of the system (thermal, electrical and water) per 1 kWh chilling energy (removed from the chilled water)? What is the ...
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Is it possible to find heat flux without emissivity value?

In this question there is a value given for absorptivity of infrared radiation however in order to work out a value for heat flux emitted from the solar panel in order to answer part (a) surely I'd ...
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Thermodynamics - heat exchanger

A thin walled double pipe counter flow heat exchanger is to be used to cool oil (Cp = 2198 J/kg°K) from 150°C to 40°C at a rate of 2.27kg/s by water (Cp = 4187 J/kg°K) that enters at 21°C at a rate of ...
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What are the factors I need to over/under estimate cost of heating in a box-like room? [closed]

For economic purposes, I am interested in studying the equilibrium of heat in a typical rectangular room from various sources, such as a classic radiator or vents. If I have a radiator sitting next to ...
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How to analyse a TEC cooler in the ansys workbench software?

I am currently doing a project on Thermoelectric Coolers . I don't have any experience in the Ansys Workbench software , but we have to do the analysis of the cooler . The device pretty much consists ...
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Furnace Combustion Air Properties

My house is heated by a condensing natural gas furnace that is rated as 92% efficient. It has the option to draw combustion air directly from outside. I know the main advantage of this is avoiding ...
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Calculate fan power

I was wondering how to calculate the required fan power for the following air pipe network. Other losses besides friction is not calculated for. Known parameters: Fan provided flow, VA = 3000 m3/h ...
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163 views

Is air a pure substance?

I was referring to some Thermodynamics text books and found that their definitions of a "pure substance" seem very subjective. "Air, for example, is a mixture of several gases, but it ...
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Thermal Resistance

I am trying to complete the following question. But feel it is not correct, so need some help. Given that heat transfer via convection off a planar wall is given by: Write out the combined thermal ...
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Joule Thompson Effect and Isenthalpic Processes

We are trying to compute the Joule Thompson (JT) coefficient for air going through a valve from P1 to P2 (where P2 is a much lower pressure). Beginning this we were thinking that the water in the air ...
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why LP gas tank does not provide fuel when the pressure reaches 10%? And to to resolve the issue?

I am having this problem that some LPG tanks don't provide fuel and other does.. I was told that the problem is whenever the ambient temperature is low (so is the gas), the pressure drops as well and ...
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Convection and radiation heat transfer coefficients

I have a problem where we have given conditions: Inner temperature is: 20 celcius Outer temperature is: -5 celcius Between these temperatures we have a wall, made of: 50mm of mineral wool with ...
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Sunlight-powered device set to change world: will it work? [closed]

Vast areas of desert sand are being shone on upon by the sun. Now that's some ENERGY! Using this proposed apparatus: Sunlight over large area is concentrated on steam chamber, where water turns into ...
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Empirical correlation for low reynolds convective heat transfer coefficient

I am looking for a heat transfer coefficient for a shape (rectangular cross section preferred, could estimate with flow over flat plates) in 2 m/s cross flow. I have found a few empirical correlations ...
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Thermodynamics question

I am looking for help with the following question please. A piston cylinder device contains 28g of saturated water vapor that is maintained at a constant pressure of 300 kPa. A resistance heater ...
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1answer
51 views

Is there a way by which one could determine the required temperature difference for stirling engine?

I am trying to make a stirling engine. I did some research work and also reviewed some books but the problem is that a great deal of information on this engine is qualitative, rather than quantitative....
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Trapping sun heat the most - greenhouse

Greenhouse have recently caught my attention lately, which yesterday I posted in Why greenhouse's inside temperature never been infinitely hot? From what I understand recently, greenhouse gases traps ...
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Rate of heat loss from a water tank

I'm doing a project for which I need to calculate heat dissipation from a water tank. I'm not well versed in thermodynamics and I can't find any place where this has been answered. I don't know the ...
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1answer
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Theoretical maximum pressure from combusting LPG

I am curious how to find or estimate the theoretical maximum pressure that the combustion of a LP gas, such as propane, can produce inside of a constant volume, such as in a cylinder. I know what the ...
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Thermodynamic energy balance

A fan that consumes 20 W of electric power when operating is claimed to discharge air from a ventilated room at a rate of 0.25 kg/s at a discharge velocity of 8 m/s. is this claim reasonable? I am ...
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steady flow energy equation [closed]

I am looking for help on the following: a) Given the system shown in the figure below, derive the steady flow energy equation from first principle. b) Again using first principles, show how the energy ...
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What is the pressure of water-dew resulted from cooling of atmospheric air down to dew point temperature?

Suppose, In a room, Air is at 1 atm presssure. So should be the pressure of any water surface in the room. If the room temperature is cooled downed to dew point temperature, vapor (moisture) will ...
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Internal Tent Temperature

Here is the problem I am working on: "Consider a perfectly sealed polygonal tent with the sun directly overhead. The solar irradiance of a surface 90° to the sun’s rays is 1,000 W/m2. However, ...
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Work calculation in a thermodynamics cycle process

To calculate work in a cyclic process (say Carnot cycle) we find area under p-v curve,or use net heat transfer= net work done. Why won't we simply add work calculated in each process of a cycle,like ...
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How is heat transfer calculated for an aqueous salt solution?

I am familiar with using $\dot{Q}=c_p\cdot\dot{m}\cdot\Delta T$ to calculate the heat transfer rate of a fluid given a singular value for specific heat capacity (such as with water), but how do I go ...
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Increase in internal energy and temperature

A banquet hall (150 m3) is to be used for a formal dinner for 20 persons. Each person occupies 0.075 m3 of space and has an average heat transfer rate of 500 kJ/hr. If the AC system fails, determine ...
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Why aircraft refrigeration uses a centrifugal compressor in place of reciprocating compressor?

Why aircraft refrigeration uses a centrifugal compressor in place of reciprocating compressor and a gas turbine in place of reciprocating expander?

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