Performance evaluation of louvered fin compact heat exchangers with vortex generators

摘要

Every day large amounts of heat are transferred in many industrial and domestic processes. This heat transfer takes place in a heat exchanger. Any energy savings in heat transfer processes have a significant impact on the fuel consumption and greenhouse gas emissions. More energy efficient heat exchangers help to meet the 20-20-20 climate and energy targets of the European Union.In many applications air is one of the working fluids (e.g. coolers in compressed air systems, heat pumps, air conditioning devices, domestic heating, etc.). When heat is exchanged with air, the main thermal resistance is located at the air side of the heat exchanger. To increase the heat transfer rate, the heat transfer surface area is enlarged by adding fins to the air side of the heat exchanger. When a high compactness is needed, complex interrupted fin surfaces are used. A typical example is the louvered fin design. The main disadvantage of the louvered fins is the high pressure drop. Delta winglets mounted on a heat transfer surface generate vortices which cause an intense mixing of the flow and thin the thermal boundary layers. In contrast to louvered fins, they enhance the heat transfer with a relatively low penalty in pressure drop.The objective of this doctoral work is to evaluate if the thermal hydraulic performance of a louvered fin heat exchanger with round tubes in a staggered layout can be improved by adding delta winglets to the fins. Such compound designs form the next generation of heat exchangers. Both experiments (flow visualizations in a water tunnel and heat transfer and pressure drop measurements in a wind tunnel) and simulations (Computational Fluid Dynamics - CFD) were performed. The louvers affect the main flow, while the delta winglets reduce the wake regions downstream of the tubes. The generated vortices cause three important mechanisms of heat transfer enhancement: a better mixing, a reduction of the thermal boundary layer thickness and a delay of the flow separation from the tube surface. Further, it was found that the vortices do not extend far downstream as they are destroyed by the deflected flow in the downstream louver bank. The compound heat exchanger has a better thermal hydraulic performance than when only vortex generators or only louvers are used. It is shown that for the same pumping power and heat duty, the compound heat exchanger is smaller in volume. Consequently, less space is required, the material cost is lower and (often also) the operational cost is reduced. The combination of louvered fins and vortex generators is mainly interesting for low Reynolds applications, such as HVAC&R applications or in compressed air systems. A well-considered location and geometry of the vortex generators are essential for an improved performance of the heat exchanger.