MODELING OF HEAT EXCHANGE IN A CONDENSATION HEAT EXCHANGER
Abstract
The scientific approach in the study is based on computational modeling of heat transfer processes in conditions of two-phase interaction of heat carriers. The study involves the use of a combination of analytical and numerical methods: computational relations for the Nusselt, Prandtl, Reynolds criteria, as well as iterative procedures using the Microsoft Excel Solver software environment [3] and the CoolProp library [4] to obtain thermodynamic properties of substances. The study of heat transfer processes in a condensation heat exchanger was carried out on the basis of computational and analytical modeling, which combines theoretical relations of heat and mass transfer with numerical optimization methods [5, 6, 7, 8].
The calculation was carried out in two main stages:
- Determination of flow parameters — flue gases and working fluid — for all boiler load modes.
At this stage, the following were determined: temperature, enthalpy, moisture content, partial pressure of water vapor, as well as the amount of heat that can be transferred from the flue gases to the working fluid.
- Modeling of heat transfer in heat exchanger zones characterized by different types of heat transfer:
- convection from the flue gas side and ammonia boiling;
- condensation of water vapor from flue gases and ammonia boiling.
For each zone, local heat transfer coefficients were determined on both sides of the pipe package - on the flue gas side and working fluid , after which the overall heat transfer coefficient was calculated.
The process parameters were refined iteratively using Solver [1] , and the thermophysical properties of substances were obtained through the CoolProp library [1].
The calculation process took into account the real geometric parameters of the heat exchanger - pipe pitches, fin sizes, fin degree, heat exchange area, etc. To check the stability of the calculation, step-by-step modeling was carried out along the rows of pipes, which allowed determining the distribution of temperatures, heat amounts, heat transfer coefficients, etc. along the entire heat exchanger.
The results obtained made it possible to assess the influence of the boiler load and the design parameters of the heat exchanger on the efficiency of heat transfer and to form a basis for further optimization of the geometry of the tube bundle, including the fin parameters.
A method for calculating heat transfer coefficients for two-phase heat exchange in a condensing heat exchanger has been developed, which takes into account the features of the ORC cycle with ammonia as the working fluid. The influence of boiler load on the ratio of convective and condensation heat exchange has been shown. The results obtained can be used to optimize the geometric parameters of the heat exchanger and the selection of pipe materials. The proposed approach is suitable for similar systems for utilizing the heat of flue gases of natural gas combustion products.
References
https://doi.org/10.20535/1813-5420.4.2021.257271.
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