In an effort to study and optimize the design of a plate heat exchanger comprising of corrugated walls with herringbone design, a CFD code is employed. Due to the difficulties induced by the geometry and flow complexity, an approach through a simplified model was followed as a first step. This simple model, comprised of only one corrugated plate and a flat plate, was constructed and simulated. The Reynolds numbers examined are 400, 900, 1000, 1150, 1250 and 1400. The SST turbulence model was preferred over other flow models for the simulation. The case where hot water (60oC) is in contact with a constant-temperature wall (20oC) was also simulated and the heat transfer rate was calculated. The results for the simplified model, presented in terms of velocity, shear stress and heat transfer coefficients, strongly encourage the simulation of one channel of the typical plate heat exchanger, i.e. the one that comprises of two corrugated plates with herringbone design having their crests nearly in contact. Preliminary results of this latter work, currently in progress, comply with visual observations.In recent years, compact heat exchangers with corrugated plates are being rapidly adopted by food and chemical process industries, replacing conventional shelland-tube exchangers. Compact heat exchangers consist of plates embossed with some form of corrugated surface pattern, usually the chevron (herringbone) geometry. The plates are assembled being abutting, with their corrugations forming narrow passages.This type of equipment offers high thermal effectiveness and close temperature approach, while allowing ease of inspection and cleaning. In order to be able to evaluate its performance, methods to predict the heat transfer coefficient and pressure drop must be developed. In this direction, CFD is considered an efficient tool for momentum and heat transfer rate estimation in this type of heat exchangers.
The type of flow in such narrow passages, which is associated with the choice of the most appropriate flow model for CFD simulation, is still an open issue in the literature. Due to the relatively high pressure drop, compared to shell-and-tube heat exchangers for equivalent flow rates, the Reynolds numbers used in this type of equipment must be lower so as the resulting pressure drops would be generally acceptable. Moreover, when this equipment is used as a reflux condenser, the limit imposed by the onset of flooding reduces the maximum Reynolds number to a value less than 2000. In a comprehensive review article concerning modelling heat transfer in narrow flow passages, state that, for the Reynolds number range of 1,500-3,000, transitional flow is expected, a kind of flow among the most difficult to simulate by conventional turbulence models.On the other hand, Shah & Wanniarachchi declare that, for the Reynolds number range 100–1500, there is evidence that the flow is already turbulent, a statement that is also supported by Vlasogiannis, whose experiments in a plate heat exchanger verify that the flow is turbulent for Re>650. Lioumbas,who studied experimentally the flow in narrow passages during counter-current gas-liquid flow, suggest that the flow exhibits the basic features of turbulent flow even for the relatively low gas Reynolds numbers tested (500<Re<1200).Focke & Knibbe, performed flow visualization experiments in narrow passages with corrugated walls. They concluded that the flow patterns in such geometries are complex, due to the existence of secondary swirling motions along the furrows of their test section and suggest that the local flow structure controls the heat transfer process in such narrow passages.The most common two-equation turbulence model, based on the equations for the turbulence energy k and its dissipation ɛ, is the k-ɛ model. To calculate the boundary layer, either “wall functions” are used, overriding the calculation of k and ɛ in the wall adjacent nodes, or integration is performed to the surface, using a “low turbulent Reynolds (low-Re) k-ɛ” model. Menter & Esch state that, in standard k-ɛ the wall shear stress and heat flux are overpredicted (especially for the lower range of the Reynolds number encountered in this kind of equipment) due to the over prediction of the turbulent length scale in the flow reattachment region, which is a characteristic phenomenon occurring on the corrugated surfaces in these geometries.Moreover, the standard k-ɛ, model requires a course grid near the wall, based on the value of y+=11, which is difficult to accomplish in confined geometries. The low-Re k-ɛ model, which uses “dumping functions” near the wall, is not considered capable of predicting the flow parameters in the complex geometry of a corrugated narrow channel, requires finer mesh near the wall, is computationally expensive compared to the standard k-ɛ model and it is unstable in convergence.
An alternative to k-ɛ model, is the k-ω model, developed by Wilcox. This model, which uses the turbulence frequency ω instead of the turbulence diffusivity ɛ, appears to be more robust, even for complex applications, and does not require very fine grid near the wall. However, it seems to be sensitive to the free stream values of turbulence frequency ω outside the boundary layer.
A combination of the two models, k-ɛ and k-ω, is the SST (Shear-Stress Transport) model, which, by employing specific “blending functions”, activates the Wilcox model near the wall and the k-ɛ model for the rest of the flow and thus it benefits from the advantages of both models. Some efforts have been made wards the effective simulation of a plate heat exchanger. Due to the modular nature of a compact heat exchanger, a common practice is to think of it as composed of a large number of unit cells (Representative Element Units, RES) and obtain results by using a single cell as the computational domain and imposing periodicity conditions across its boundaries.Although compact heat exchangers with corrugated plates offer many advantages compared to conventional heat exchangers, their main drawback is the absence of a general design method. The variation of their basic geometric details (i.e. aspect ratio, shape and angle of the corrugations) produces various design configurations, but this variety, although it increases the ability of compact heat exchangers to adapt to different applications, renders it very difficult to generate an adequate ‘database’ covering all possible configurations. Thus, CFD simulation is promising in this respect, as it allows computation for various geometries, and study of the effect of various design configurations on heat transfer and flow characteristics.In an effort to investigate the complex flow and heat transfer inside this equipment, this work starts by simulating and studying a simplified channel and, after gaining adequate experience, it continues by the CFD simulation of a module of a compact heat exchanger consisting of two corrugated plates. The data acquired from former simulation is consistent with the single corrugated plate results and verifies the importance of corrugations on both flow distribution and heat transfer rate. To compensate for the limited experimental data concerning the flow and heat transfer characteristics, the results are validated by comparing the overall Nusselt numbers calculated for this simple channel to those of a commercial heat exchanger and are found to be in reasonably good agreement. In addition, the results of the simulation of a complete heat exchanger agree with the visual observations in similar geometries.
Since the simulation is computationally intensive, it is necessary to employ a cluster of parallel workstations, in order to use finer grid and more appropriate CFD flow models. The results of this study, apart from enhancing our physical understanding of the flow inside compact heat exchangers, can also contribute to the formulation of design equations that could be appended to commercial process simulators.Additional experimental work is needed to validate and support CFD results, and towards this direction there is work in progress on visualization and measurements of pressure drop, local velocity profiles and heat transfer coefficients in this type of equipment.
