Selection of qualitative parameters in heat exchanger calculations!
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This post was last edited by YunDanFengYeQing on 2018-10-23 at 13:23. During the calculation process of heat exchangers, after determining the characteristic temperature, parameters such as the specific heat, dynamic viscosity, and density of the medium are selected. This applies to situations where the pressure remains constant on that side. Currently, we’ve encountered a problem in our calculations: the medium on that side (liquid nitrogen) enters a constant-volume chamber for heat exchange (it enters at atmospheric pressure and -196°C, and after heat exchange, its pressure reaches 40 MPa). As a result, both its pressure and temperature are constantly changing (as it absorbs heat, its temperature and pressure rise). Thus, while we can calculate the characteristic temperature as required during our thermodynamic calculations, is there also a concept of “characteristic pressure”? How should we select the parameters for the medium on that side?
Correct answer: If physical properties are related to pressure (such as in gases), then there is a \"qualitative pressure\" – the average pressure at the inlet and outlet.
Very good, you have now delved into the most complex aspects of heat transfer calculations. There are also several fluids whose physical properties do not change with temperature and pressure. What to do when the physical properties do change with temperature and pressure, but the outlet temperature and pressure are unknown? Then an iterative approach must be used: first assume the exit temperature, then determine the qualitative material properties, calculate the thermal equilibrium to establish the exit temperature, adjust the qualitative properties, and recalculate – repeating this process until the difference between the two results meets the desired accuracy level. When the number of iterations is large, only a computer can be used for the calculation.
Thank you! Is the setting of the qualitative pressure done as described by the teacher upstairs?
This post was last edited by YunDanFengYeQing on 2018-10-23 at 10:27. Actually, this is a special operating condition; I’ve never seen any similar methods or cases before. I’m bringing this up here so that everyone can analyze it； Liquid nitrogen at normal pressure (-196°C) is introduced into a piston cylinder. Once the liquid nitrogen enters, the piston cylinder becomes sealed, and the piston section (in which the liquid nitrogen is contained) moves within the cylinder; heating is applied to this piston section during its movement. What happens to the liquid nitrogen inside the piston section? Should heating and pressurization be carried out by following the isochoric lines on the enthalpy diagram?
If the substance inside the chamber is uniform, its density remains constant; heating and pressurization occur along isochoric lines. However, if there is a mixture of gas, liquid, or solid, the situation becomes complex and difficult to analyze.
The issues mentioned on the 2nd floor. I’ve never seen this kind of operating condition. What application needs this? What material piston can prevent liquid nitrogen from leaking and avoid damage in the event of a leak? The heat transfer in this case should differ significantly from the heat transfer calculations used for conventional heat exchangers. It can be regarded as a mixture of steady state and transient. If the current steady-state calculation method is used for heat exchange, the deviation may be quite large. And for transients, there might not even be a reference formula. In general engineering, simulation tests are first conducted to determine certain computational parameters qualitatively. Regarding your question, liquid nitrogen should have the same effect as heating in a sealed tank. However, this heat transfer process should be considered as enhanced heat transfer; many parameters such as the heat transfer area, flow velocity inside the tubes, and flow rate are different from those in normal steady-state calculations.
Thank you for your answer! The material of the piston ring is polytrifluorochloroethylene, and a dynamic sealing ring is used in the piston cylinder to achieve sealing. This sealing structure has been tested with liquid nitrogen under static conditions, and its sealing performance meets the required standards; however, issues such as heat transfer calculations and the variation curves associated with liquid nitrogen cannot be determined. We are doing this to develop a patented device; there is probably no such application available at the moment.