(568b) Evaluation of the Boundary Conditions in Cfd Modeling of Heat Transfer in the 3d Chevron Type Plate Heat Exchanger

New heat exchanger geometries are traditionally developed by the trial and error method using some kind of heuristics. Theoretical predictions of the thermal efficiency of plate heat exchangers would facilitate design of new heat exchangers. Also accurate prediction of reactions, heat transfer and fluid flow in different heat exchanger geometries would help to minimize fouling of heat exchanges by geometrical changes.

The objective of this work was to model fluid flow and heat transfer in the corrugated 3D plate heat exchanger geometry with a computational fluid dynamics (CFD) program, Fluent 6.1, and to find the most realistic heat transfer boundary conditions for a plate heat exchanger and to evaluate the limitations of different boundary conditions.

The plate heat exchanger studied was chevron type (M15-M) with corrugation angle of 60 ° and it was made by Alfa Laval. The plate heat exchanger consists of several thin, corrugated plates, which are compressed together and sealed with gaskets. In every second plate the corrugated herringbone pattern goes upwards and in every second downwards and hence complicated passages are formed between plates. A corrugated flow channel generates vortices even with low flow velocities. In that case mixing of the fluid is increased and thus heat transfer becomes more effective. In the cannels warm and cold flows alternate and heat transfers through the plates by conduction. Heat transfer by forced convection also exists due to the fluid flow. Heat transfer by radiation can be neglected because temperature in the plate heat exchanger studied is quite low (max. 378 K). Accurate modeling of the whole plate heat exchanger with CFD is not possible because of limited computational capacity. For the modeling a small part of the heat exchanger structure should be selected which describes the physical phenomena to be modeled. Thus a flow channel between two plates of dimensions of 0.32 m x 0.145 m was chosen to modeling, which was meshed to 240 000 unstructured elements since structured mesh was not possible to generate because of the very complex geometry. For the construction material physical properties of titanium and for the fluid properties physical properties of water were used.

Fluid flow at the geometry was modeled using the Navier-Stokes equations. Reynolds number based on the mean hydraulic diameter of the flow channel is between 1800 and 2300, which indicates that the flow field is not fully laminar. The direct numerical simulation was computationally quite heavy to perform. The flow field obtained follows qualitatively the flow presented in the theory, which means that both crossing and longitudinal, wavy flows can be seen in the flow field [1].

In this study different heat transfer boundary conditions of the commercial computational fluid dynamics (CFD) program, Fluent 6.1, were tested and their applicability in modeling complicated heat transfer geometry was investigated and discussed. Build-in boundary conditions of Fluent 6.1 available for this case are Heat flux, Convection and Constant wall temperature. At the Convection boundary condition the values of the outer fluid heat transfer coefficient and temperature are defined. While using the Heat flux boundary condition an appropriate value for the heat flux through the wall is defined. At the Constant wall temperature boundary condition the temperature of the wall is defined. According to those definitions program calculates the temperature field in the geometry. [2] With the geometry used none of the three alternative thermal boundary conditions describes exactly the physical situation in the plate heat exchanger.

While using the Constant wall temperature boundary condition, the temperature of the plate was defined and assumed to be 353 K, the same as the average temperature of the process fluid flowing outside of the geometry. The heat flux through the wall and the temperature field in the geometry are computed. Temperature defined constant at walls is definitely an inaccurate assumption in this case of heat exchanger because temperature has difference between the inlet and outlet or at least it has local variations caused by corrugations.

With the Heat flux boundary condition the heat flux had to be defined as a constant at wall. In this study empirical Nusselt number correlation with the overall mass and heat balances was used to estimate the heat flux [3]. The local heat transfer coefficient varies spatially at the wall. Thus the overall heat transfer coefficient is not an exact approximation. Furthermore to design a new heat exchanger geometry it would be beneficial to calculate the heat flux in order to find out performance of the structure, not to define it.

While using the Convection boundary condition the temperature and the heat transfer coefficient of surroundings need to be defined as constants. The surrounding fluid has spatial temperature variations similar as flow inside the geometry. However, the heat transfer coefficient is computed for the fluid inside the modeled channel which means that its dependence on temperature and flow rate is being taken into account.

Deficiencies were found out in all the three heat transfer boundary conditions studied. To model the heat transfer with CFD in the plate heat exchanger is problematic because of the assumptions that have to be made when defining the boundary conditions. Because of the very complex geometry, values of those parameters are function of the place and can not be defined unambiguously. However, the Convection boundary condition describes most reliably the physical situation of heat transfer in the geometry studied.

References:

[1] Martin H (1996) A theoretical approach to predict the performance of chevron-type plate heat exchangers. Chemical engineering and processing, 35, (4), s. 301 ? 310.

[2] Fluent 6.1 (2003) User's guide. Lebanon, Fluent Inc.

[3] Riihimäki M, Muurinen E, Keiski RL (2004) Thermal analysis of a plate heat exchanger in fouling conditions ? Analysis of process information and calculations based on design equations. 16th International Congress of Chemical and Process Engineering, 22.?26.8.2004, Prague, Czech Republic.

Key words: boundary condition, CFD, heat transfer, modeling, plate heat exchanger.