Seeking the optimum solution for the design of a typical PHE
Multi-scale and/or multi-disciplinary approach to process-product innovation
CFD & Multiscale Modelling in Chemical Engineering (T3-4P)
Keywords: Plate heat exchanger, CFD, optimization
In the last decades, the need for compact yet efficient heat exchange equipment led to the development in the design of plate heat exchangers (PHE) and to their use in a wide range of industrial applications, since they hold advantages over conventional equipment. A common type of PHE comprises some kind of near-sinusoidal corrugations, which form a herringbone pattern; when two of these plates are placed abutting, a channel with complicated passages is formed. Thus, as the fluid flows inside this channel, it undergoes a series of periodic changes of its direction as it encounters successive peaks and valleys. This type of flow augments heat transfer, due to flow separation and reattachment that causes the “restart” of the boundary layer (Taslim & Kercher, 1996). On the other hand, the complexity induced by the corrugations significantly increases the friction losses.
The limited published data concerning the performance of PHEs has hindered the development of a generic design model for the corrugated plates of a PHE. This is probably due to the highly competitive market (Heavner & Kumar, 1993) that renders data regarding this type of equipment proprietary. The available experimental data that deal with the effect of various geometrical parameters on the heat transfer coefficient, the friction factor and the flow field in general (Taslim & Kercher, 1996; Heavner & Kumar, 1993; Mehrabian & Poulter, 2000; Sparrow & Comb, 1983), concern plate configurations with certain geometrical characteristics. Moreover, the available data from numerical simulations, deals mostly with Representative Elements (e.g. the flow field inside a single furrow) (Mehrabian & Poulter, 2000). In our Laboratory, by studying a typical commercial PHE both experimentally and numerically (Kanaris et al., 2005; Kanaris et al., 2006), it is proved that CFD is an effective and reliable tool to predict flow characteristics and to estimate heat transfer rates, as well as pressure losses, in this type of process equipment. Thus, to optimize the design of the corrugated plates, the various geometrical design parameters that influence the thermohydraulic features of the flow can be numerically studied and their effect on the overall performance of a PHE can be estimated. Among the most important design parameters are the so called “angle of attack” or “chevron angle”, which determines the performance of a given plate (Heavner & Kumar, 1993), the inter-wall spacing, which influences the entrance region heat transfer coefficients (Sparrow & Comb, 1983), as well as the roundness of the corrugations, which affects the pressure drop (Kanaris et al., 2006).
It is expected that the correct interpretation of the numerical (CFD) results will lead to the formulation of generic design correlations that would maximize the PHE performance by compromising between the heat transfer augmentation and friction losses. These correlations, combined with a weighting factor expressing a compromise between friction losses and heat transfer augmentation, will lead to the formulation of an objective function that would optimize the energy economy of the plate heat exchanger. This work, currently in progress, uses both simulation results and relevant experimental data in order to create an algebraic metamodel to be subsequently optimized. As the underlying phenomena are expected to be non-linear, the use of global optimization techniques for identifying a true optimal solution of the metamodel is necessary. This challenging optimization problem is tackled through the use of a global optimization system (Tawarmalani & Sahinidis, 2004). Simulation and optimization are used iteratively, with the use of a surrogate function. This function will then be optimized and the solution will be used to guide additional simulations in the vicinity of the global solution of the current metamodel and, eventually, will lead to the validation of the optimal solution.
References
• Heavner, R.L. and Kumar, H., 1993. Performance of an Industrial Plate Heat Exchanger: Effect of Chevron Angle. In: AIChE Symposium Series, 89, 295:262-267.
• Kanaris, A.G., Mouza, A.A. and Paras, S.V., 2005. Flow and heat transfer in narrow channels with corrugated walls: a CFD code application. Chemical Engineering Research & Design, 83 (A5), 460-468, 2005
• Kanaris, A.G., Mouza, A.A. and Paras, S.V., 2006. Flow and heat transfer prediction in a corrugated plate heat exchanger using a CFD code. Chemical Engineering and Technology, 29, 8: 923-930.
• Mehrabian, M.A. and Poulter, R., 2000. Hydrodynamics and thermal characteristics of corrugated channels: computational approach. Applied Mathematical Modeling, 24, 5:343-364.
• Sparrow, E.M. and Comb, J.W., 1983. Effect of interwall spacing and fluid flow inlet conditions on a corrugated-wall heat exchanger. Int. J. Heat Mass Transfer, 26, 7: 993-1005.
• Taslim, M.E. and Kercher, D.M., 1996. Experimental Heat Transfer and Friction in Channels Roughened With Angled, V-Shaped, and Discrete Ribs on Two Opposite Walls. In: Transactions of the ASME, 118, 20: pp. 20-28.
• Tawarmalani, M. and Sahinidis, N.V., 2004. Global optimization of mixed-integer nonlinear programs: A theoretical and computational study. Mathematical Programming, 99, 3: 563-591.
Presented Tuesday 18, 13:30 to 15:00, in session CFD & Mutliscale Modelling in Chemical Engineering (T3-4P).
