What will happen to the heat input of the two-phase system?

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Discussion Overview

The discussion revolves around the heat input of a two-phase system involving a low boiling point liquid in a sealed tank placed in a dry oven. Participants explore the effects of temperature, pressure, and phase changes on heat input calculations, considering various factors such as heat conduction, convection, and the properties of the gas phase.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether it is appropriate to ignore heat convection and focus solely on heat conduction for calculating heat input in the system.
  • Another participant raises the issue of heat of evaporation and its relation to the internal energy of the system, suggesting that knowledge of heat input is necessary to determine it.
  • A different participant argues that if the system reaches equilibrium, the liquid will evaporate until either all liquid is vapor or the vapor pressure equals the equilibrium vapor pressure at the oven temperature, indicating a method to calculate heat input for vaporization.
  • There is a suggestion that in the absence of vapor property information, one could assume the vapor behaves as an ideal gas to estimate its properties.

Areas of Agreement / Disagreement

Participants express differing views on the calculation of heat input, particularly regarding the roles of convection and conduction, as well as the necessity of knowing vapor properties. The discussion remains unresolved with multiple competing perspectives on these aspects.

Contextual Notes

Participants note the variability of density and specific heat capacity with temperature, which may complicate calculations. There is also uncertainty regarding the properties of the gas phase when no information is available.

Who May Find This Useful

This discussion may be of interest to those studying thermodynamics, phase transitions, or heat transfer in two-phase systems, particularly in experimental or applied contexts.

nesca
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If I have a tank half-filled with low boiling point liquid at ambient temperature, then this tank is placed inside dry oven whose temperature is kept to be around 5℃ above the liquid boiling temperature. A fan is placed inside the dry oven as an effort to make temperature distribution even or at least minimize large gap to happen. Because of this the liquid temperature is increased, yet there is no bubble developed, or in other words there is no liquid bulk motion inside the tank.

After boiling temperature is reached, the tank is tightly sealed, thus the pressure is increased due to liquid evaporation.

In this case, to calculate heat input, is it okay to ignore heat convection both in liquid and gas and focus with heat conduction only?

Is the equation below can be used for heat input calculation?

Q=mcΔT

What if the density and liquid specific capacity vary with temperature? How to determine the mass and specific capacity?

If the gas phase is equal to liquid vapor and there is no information about vapor properties, what is the best way to at least obtain the density, thermal conductivity, and heat capacity for the gas phase?
 
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What about the heat of evaporation?
 
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mjc123 said:
What about the heat of evaporation?
If I'm not mistaken, to determine the heat of evaporation, we have to obtain internal energy of the system, and to do it ones should know the heat input of the system, shouldn't they?
 
No, if the system reaches equilibrium, the liquid will evaporate either until it is all vapour, or until the vapour pressure equals the equilibrium vapour pressure at the oven temperature (depending on how much liquid there is and the volume of the oven). You can work out how much liquid evaporates, and knowing the molar (or specific) heat of vaporisation, you can work out the heat input for vaporisation.
If you have no information on vapour properties, you can as a first approximation assume it's an ideal gas and calculate what its properties would be.
 
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