Thermo-hydraulic study of inclined louvered fins

Heat transfer is a naturally occurring process. It is used in a multitude of technical and domestic applications such as climate control, preparation and conservation of food, production of electricity. . . As such heat transfer affects every facet of our daily lives. Over the past decades the worldwide energy consumption has increased considerably. This is due to the increased standard of living and an overall higher demand for comfort. Improving the energy efficiency of technical processes and applications will help reduce the increasing energy demand. Because of the enormous amount of energy transferred through heat transfer, further improvements in this field can have a major impact. This research project focuses on space heating and air conditioning, which contributes a large share of the total domestic energy consumption. The commonly used heat exchanger design for this application is a crossflow fin-and-tube heat exchanger. When exchanging heat with air, the main resistance to heat transfer is located on the air side. To increase the heat transfer rate fins are added on the airside which increase the surface area. Over the past decades a considerable amount of research has been performed to improve the performance of fin-andtube heat exchangers: e.g. transfer the same amount of heat with a smaller volume (automotive applications). The heat transfer resistance is very closely linked with the flow mechanics inside the heat exchanger and this through the concept of the thermal boundary layer. Reducing the thermal boundary layer thickness will result in an increased heat transfer rate. In the first chapter, an overview is presented of the evolution of the fin-and-tube heat exchanger over the past decades, focused upon the ideas used to reduce the boundary layer thickness. Dividing the fin surface into smaller units, louvers, has proven to be very effective. These interrupted fin designs result in increased heat transfer due to boundary layer restarting and at higher Reynolds numbers they promote self-sustained flow unsteadiness which further increases the heat transfer rate. Two commonly used interrupted fin designs are the slit fin (also called the offset strip fin) and the louvered fin. Today louvered fins are the most used type for automotive air conditioning applications. These fins provide a very compact heat eexchanger with a large heat transfer rate per m3 but also result in a large pressure drop on the airside. This is because at high Reynolds numbers the flow is deflected inside the fin pattern to become aligned with the louvers, ‘louver oriented flow’. This extends the flow path, increasing contact time between the fluid and the fin which results in a higher heat transfer rate but also increases the frictional pressure drop. At low Reynolds numbers (less than 200 when based on the louver pitch) the flow is no longer aligned with the louvers, and the heat transfer coefficients decrease significantly. In this research project the inclined louvered fin design, a hybrid between the slit fin and louvered fin is studied. The aim of this study is to determine the thermo-hydraulic behaviour, focused on the low Reynolds numbers and to determine the impact of the geometric parameters on the performance. A lot of attention is given to the interaction between the flow and the thermo-hydraulics. In Chapter 2 the basic geometry of the inclined louvered fin is introduced. A literature survey revealed that no data had been published on this design. Only two authors had previously provided thermo-hydraulic data on related esigns and this data showed an improved heat transfer rate and reduced pressure drop compared to the louvered fin design. To determine which of the geometric parameters have a significant impact on the thermo-hydraulic behaviour, a numerical screening experiment (CFD) was performed. The geometric ranges were inspired by previous studies on the louvered and slit fin designs. The screening showed that the Reynolds number, fin pitch ratio (ratio of the fin pitch to the louver length) and fin angle had the strongest impact on the heat transfer and pressure drop. These parameters were thus studied in more detail through experiments. One of the key ideas behind the inclined louvered fin design is to generate a flow deflection, just as in louvered fin, but to do so at low Reynolds numbers, increasing the contact time and thus hopefully the heat transfer rate. This deflection arises due to the boundary layer growth: thick boundary layers will block flow passages and force the flow to wind up and down. To study the flow behaviour the flow was visualised in a water tunnel by using dye injection, as described in Chapter 3. The presence of flow deflection at low Reynolds numbers was confirmed. Five different configurations were studied, showing the geometry had a very large impact on the flow behaviour. To quantify the mean flow behaviour, the fin angle alignment factor was introduced. By using a combination of experimental data (flow visualisation) and numerical data (CFD) the flow behaviour was studied over a range of Reynolds numbers and geometric parameters. A simple geometric model based on laminar boundary layer growth proved very effective at predicting based on the geometry and the Reynolds number. The flow visualisation also revealed that the flow became periodic unsteady at low Reynolds numbers (for some configurations in the order of 250 when expressed using the louver pitch). This transition is geometrically triggered due to recirculation zones on the inclined parts of the inlet, turnaround and exit louver. The transition from steady to unsteady flow is described in Chapter 4 using numerical simulations combined with visual observations. To determine the pressure drop and heat transfer coefficients, scaled up models (20:1) of the inclined louvered fin were tested in an open wind tunnel test setup. The test rig, measurement procedure and data reduction are described in Chapter 5. To determine the heat transfer coefficient a uniform heat flux boundary was imposed (through resistor heating) and local wall temperatures were measured using thermocouples. Pressure drop measurements were done for 9 configurations over a wide range of Reynolds numbers. This experimental data was combined with numerical data for the lowest Reynolds numbers. A thorough trend analysis was performed on this dataset, confirming the results of the screening. The friction curves of the configurations with the smallest fin angle showed two clear points of transition which could be linked to the change in flow deflection and the transition to unsteady flow respectively. For the configurations with higher fin angles the friction factor curves only presented one transition point linked to transition to unsteady flow. The first transition point would occur at very low Reynolds numbers. A comparison of the friction data to data determined from scaled models of louvered, slit and convex louvered fins showed that the shape of the turnaround louver has a strong impact on the resulting friction factor. Chapter 6 presents the overall conclusions of this work and some recommendations for future research projects