A compact heat exchanger is a device used to transfer thermal energy between two or more fluids. The most extensive use of compact heat exchangers occurs in the commercial trucking industry. Most compact heat exchanger designs contain tubes carrying one fluid and external fins through which passes another fluid. To enhance the fin-side heat transfer in a compact heat exchanger, which is typically the air side of the heat exchanger, louvers are manufactured into the fins. Louvered fins initiate the growth of new boundary layers such that the average convective heat transfer coefficient is higher than that which would occur for a continuous fin. Approximately 85% of the total thermal resistance occurs on the air side of the heat exchanger. To design more space and weight efficient heat exchangers, it is imperative to gain a fundamental understanding of the mechanisms that serve to increase the heat transfer on the air side. This thesis presents the heat transfer results of three scaled-up louvered fin geometries and compares these results to six additional models in which the louver angle, fin pitch and Reynolds number were varied. Two experiments were performed to determine the reference temperature used for the calculation of the heat transfer coefficients. The use of two reference temperatures allowed the effects of the flow field and thermal field to be separated. This thesis also presents details of an optimization study performed for a louvered fin array. The results of the experimental study showed that the hot thermal wakes formed at the entrance louver have an adverse effect on the heat transfer of downstream louvers. Measuring the adiabatic wall temperature of the louvers in the array showed the effect of these thermal wakes. The experimental study showed that the optimal louver geometry was Reynolds number dependent. For the lower two Reynolds numbers of ReLp = 230 and 370, the Fp/Lp = 1.52, q = 27° model was found to be the best performer, which does not agree with previous studies. For ReLp = 1016, the Fp/Lp = 0.91, q = 39° model was shown to have optimal heat transfer performance, which is in agreement with a previous study performed by Chang and Wang (1996). / Master of Science

Understanding the mechanisms that serve to increase heat transfer provides valuable knowledge to minimize the size and maximize the performance of compact heat exchangers. This document presents a detailed experimental heat transfer study of six scaled up louvered fin geometries that are typical of those found in modern louvered fin compact heat exchangers. Heat transfer measurements were performed over a range of Reynolds numbers and with two different boundary conditions. A fully heated boundary condition allowed the effects of the thermal field to be observed while an adiabatic boundary condition allowed the effects of the flow field to be observed. The results indicated that the complex thermal and flow field patterns that developed within the louvered fin geometries strongly affected the heat transfer of individual louvers. In the entrance region of the louvered array, the effects of the flow field were dominant while in the fully developed region of the louvered arrays, the effects of the thermal field were dominant. A companion two-dimensional CFD study indicated that the heat transfer trends of the louvers resulting from both the thermal and flow fields were well predicted. Based on heat transfer performance, it was determined that the theta = 27°, Fp/Lp = 1.52 geometry performed the best at Re = 230 and Re = 370, while the theta = 39°, Fp/Lp = 0.91 geometry performed best at Re = 1016. / Master of Science

There are several different types of compact heat exchangers used in applications where small size and weight are required. One particular type of compact heat exchanger, the louvered fin heat exchanger, has been used heavily in the automotive and air conditioning industries. Over the last several decades, the majority of the work towards improving louvered fin exchanger efficiency has focused on designing more efficient fins by optimizing fin parameters like louver angle, fin pitch, louver pitch, and louver length. At this point in time, many improvements to standard louver geometry have been made, so other surfaces and methods of enhancing exchanger performance need to be studied if any significant future efficiency gains are to be expected. This thesis presents a detailed experimental study that has two major foci relative to the performance of the louvered fin compact heat exchanger. The first is to determine the effect of louver length on pressure drop and tube wall heat transfer, which is the primary heat transfer surface in the heat exchanger. The second is to augment tube wall heat transfer with the use of delta winglets placed on the fins near the tube wall. These studies were completed on a 20X scale model of a louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°, over a Reynolds number range based on louver pitch of 230 < ReLp < 1016. The three louver lengths evaluated were 100%, 82%, and 70% of the fin height and delta winglet experiments were performed for louver length to fin pitch ratios of 100% and 70%. Heat transfer results for the louver length tests show that decreasing louver length leads to increases in tube wall heat transfer of 0% to 50% depending on Reynolds number. Also, delta winglets placed on the fins near the tube wall have been shown to produce average tube wall heat transfer augmentations of up to 52%. / Master of Science

Compact heat exchangers are widely used by the automotive industry in systems that cool engine components. Louvered fin heat exchangers are used over their continuous fin counterparts because of the significant advantages they provide in heat transfer efficiency, while only causing small increases in overall pressure losses. With the recent emphasis that has been placed on reducing fuel consumption, decreasing the size of the compact heat exchanger has become an important concern. With reduction in size comes not only weight savings, but also a decrease in frontal area in a vehicle that must be dedicated to the heat exchanger, allowing for more aerodynamic vehicle designs. Air-side resistance on the tube wall and louvered fin surfaces comprises over 85% of total resistance to heat transfer in louvered fin heat exchangers. The tube wall surface is considered the primary surface for heat transfer, where the temperature between the working fluid and convecting air is at a maximum. Recent studies have shown that implementing delta winglets on louvered fins along the tube wall is an effective method of augmenting tube wall heat transfer. In this thesis, the effect of delta winglets is investigated in both two- and three-dimensional louvered fin arrays. For both geometries, winglets are simulated in a manufacturable configuration, where piercings in the louvered fins that would result from the winglet manufacturing process are modeled. Using the two-dimensional geometry to model tube wall heat transfer was shown not to accurately predict heat transfer coefficients. In a two-dimensional geometry, winglets were found not to be an effective means for augmenting tube wall heat transfer and caused only 8% augmentation. Using the three-dimensional geometry, winglets with simulated piercings were observed to cause up to 24% tube wall heat transfer augmentation, with a corresponding increase in pressure losses of only 10%. / Master of Science