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Description
Perforated plate heat exchangers (PPHEs) are made of alternately arranged high thermal conductivity plates with an insulating spacer in between. While the plates help in inter-fluid heat exchange in the lateral direction, the spacer minimizes the axial conduction losses. They, being very compact and efficient, find use in applications in many places including space, 2K helium cryogenic and other low temperature heat transfer applications.
Performance predictions of PPHEs are usually done numerically through CFD simulations. However, a major issue in CFD modelling is the meshing of a large number of plates with fine pores (pore diameter typically 0.5 to 0.8 mm) involving a substantial increase in the number of computational cells and inviting challenges in computation procedure and time. One of the effective ways of numerical modelling of PPHEs is that the perforated plates are considered to be an anisotropic porous medium with unidirectional permeability. However, certain aspects or issues need to be resolved while using the porous medium method in modelling a PPHE. In this paper, these issues under varying geometrical orientations and dimensions are resolved and presented.
Porous medium modelling of a PPHE using a finite volume method such as Ansys FluentTM requires inputs such as viscous resistance, inertia resistance and porosity of the body. Thermal simulation can be done using either local thermal equilibrium (LTE) or local thermal non-equilibrium (LTNE) approach. As reported, the thermal non-equilibrium approach provides better results, hence, in the present case, we consider this approach. For LTNE model we require the local heat transfer coefficient data. The correlations for the Colburn factor are available in the literature for the Reynolds number 10<Re<1000. In some applications where pressure drop is not a critical criterion, a smaller flow area is preferred which may lead to a Reynolds number (Re) greater than 1000. Numerically obtained correlations for Colburn factor and friction factor for 1000<Re<4000 as functions of geometrical parameters are presented in the paper.
For calculating pressure drop across the stack of perforated plates and spacers Darcy-Forchheimer equation has been used. To calculate the viscous and inertial resistance, permeability and Forchheimer coefficient have been calculated by using correlations from the Bae & Kim and the Li models. Many researchers have worked on a single perforated plate to calculate the pressure drop across it, but very little work has been reported on the pressure drop across a perforated plate heat exchanger. In a perforated plate heat exchanger, plates are arranged either in an aligned hole (inline) manner or shifted-holes manner. In the case of inline-holes arrangement, because of the small spacer thickness, we consider interconnected pores where inertia loss is taken zero. For shifted-holes arrangement, the magnitude of velocity and direction change significantly in between the plates and, hence, the inertia loss is accounted for in the Darcy-Forchheimer equation. Steady, pressure-based, incompressible flow has been used for the study. The numerical simulations consider various parameters such as plate porosity, thickness of the plates, pore diameter, plate material, fluid properties to comprehensively investigate the flow friction behaviour.
For validation of the proposed model, PPHEs, for which experimental data are available, have been modelled under similar operating conditions using the porous medium method. The results compare well within the acceptable limit. Some corrective measures for further improvements are also in consideration. This model can be effectively used for performance prediction of PPHE covering a wide range of geometrical parameters and Reynolds number.
| Submitters Country | INDIA |
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