Numerical investigation of heat transfer and pressure loss of flow through a heated plate mounted by perforated concave rectangular winglet vortex generators in a channel

The low thermal conductivity of air in fin-and-tube heat exchangers causes high thermal resistance of the air side and results in a low heat transfer rate. This heat transfer rate on the air side can be improved by increasing the heat transfer coefficient. One way to increase the heat transfer coefficient on the air side is to use a vortex generator (VG), which can generate longitudinal vortex (LV) increasing fluid mixing. Therefore, this study aims to numerically analyze heat transfer characteristics and pressure drop of airflow through a heated plate by installing VG in a rectangular channel. Vortex generators (VGs) used in numerical modeling are rectangular winglet pairs (RWPs) and concave rectangular winglet pairs (CRWPs) with 30 attack angle. The number of pairs of VG is varied by one, two, and three with/without holes. The velocity of airflow varies in the range of 0.4-2.0 m/s at intervals of 0.2 m/s. The simulation results show that in the configuration of the three pairs of VG, the decrease in the convection heat transfer coefficient in the case of the perforated CRWP is 3.98% of the CRWP without holes at a velocity of 2.0 m/s. While in the configuration of three pairs of perforated RWP VGs, the decrease in convection heat transfer coefficient is 5.87% from RWP without holes at a velocity of 2.0 m/s. In the configuration of three pairs of perforated VGs at the highest velocity, the decrease in pressure drop in the CRWP and RWP cases is 30.73% and 13.87% of the VGs without holes, respectively

Heat transfer characteristics of plate fin heat sink with longitudinal vortex generators

Purpose This study aims to provide an insight into the relationship between design parameters and thermal performance of plate fin heat sinks (PFHSs) incorporating longitudinal vortex generators (VGs) inside a PFHS channel. Design/methodology/approach A computational fluid dynamics model of a delta winglet pair VG mounted inside a PFHS geometry is detailed, and the model is validated by comparison with experimental data. The validated model is used to perform a virtual design of experiments study of the heat sink with bottom plate and vertical plate mounted VGs. Data from this study is used to regress a response surface enabling the influence of each of the assessed design variables on thermal performance and flow resistance to be determined. Findings The results of this study show that the thermal hydraulic performances of a PFHS with bottom plate mounted VG and vertical plate fin mounted VG are, respectively, 1.12 and 1.17 times higher than the baseline PFHS. Further, the performance variation of the heat sink with VG, relative to delta winglet’s arrangement (common flow up and common flow down), trailing edge gap length and Reynolds number were also evaluated and reported. Originality/value For the first time, performance characteristics of delta winglet VGs mounted inside the PFHS are evaluated against different design variables and a polynomial regression model is developed. The developed regression model and computed results can be used to design high performance PFHSs mounted with delta winglet VGs

Evaluation of thermal and hydraulic of air flow through perforated concave delta winglet vortex generators in a rectangular channel with field synergy principle

A compact heat exchanger can be found in air conditioning, automotive industry, chemical processing, etc. Most compact heat exchangers use gas as a heating or cooling fluid. However, gas has high thermal resistance, which affects lower heat transfer. In order to reduce thermal resistance on the gas side, the convection heat transfer coefficient is increased. One effective way to enhance the convection heat transfer coefficient is to use a vortex generator. Vortex generators are surface protrusions that are able to manipulate flow resulting in an increase in convection heat transfer coefficient by enhancing the mixture of air near the wall with the air in the main flow. Therefore, this work aims to evaluate the thermal and hydraulic characteristics of airflow through the perforated concave delta winglet vortex generator. This study was conducted on delta winglet vortex generators (DW VGs) and concave delta winglet vortex generator (CDW VGs) with the 45 angle of attack with a number of hole three-holes that applied on every vortex generator with one-line fitting, two-line fitting, and three-line fitting respectively. Results of simulation revealed that heat transfer coefficient (h) for perforated CDW VGs decrease 16.07% and pressure drop decrease 7% compare to that without hole configuration at Reynolds number of 8600. Convection heat transfer coefficient for perforated DW VGs decrease 13.76% and pressure drop decrease 5.22% compare to delta winglet without hole at Reynolds number of 8600

Numerical Study of Thermal Performance Improvement By Novel Structures in the Building Energy Storage Systems

In this work, numerical studies were conducted to investigate the effectiveness of two fin-like novel structures used for heat-transfer enhancement in two building energy storage systems including thermal energy storage and battery energy storage. Firstly, thin layer ring structure was numerically investigated for thermal performance improvement in the thermal energy storage. From the results obtained in this study, the area ratio can be increased by 4% when using the thin layer ring during the same time period. The thin layer ring structure can shorten ice formation period and increase its efficiency. Further study was conducted for the factorial analysis of three parameters, including thickness, material and arrangement of thin layer ring. From the results, it shows that ice formation period can be shortened with the increase of conductivity and area of thin layer ring, while it is also dependent on thickness. Using Taguchi method, the statistic results show that material has the greatest impact on ice increased area. After that, arrangement has relatively less influence on ice increased area. However, thickness has the trifling effect on ice-increased area. The optimal combination of each factor (parameter) has been determined, and the optimal condition is A3B2C1. That is to say, for material = copper, thickness = 1mm，and arrangement = staggered, the best result of heat-transfer enhancement was obtained among all the cases studied. The reproducibility of these conditions has been verified by two analytical results. Secondly, in battery energy storage, numerical simulations have been conducted to explore the air flow and heat transfer at different discharging rates in a horizontal rectangular channel with two different configurations of vortex generator (VG), such as rectangular rib and delta winglet. The simulation in air flow domain with characteristics of heat transfer and flow structure show that both types of vortex generators can enhance heat transfer before VGs, but only delta winglet VG can still enhance local heat transfer after it due to more vortices generated that can mix cold and hot air flow between the top and bottom thermal layers completely. The encouraging result shows that the maximum temperature of pouch cell can be decreased more by delta winglet than by rectangular rib. For the discharging rate at 5C, it can be decreased by 10% and the local Nusselt number can be increased by 38% compared to the baseline scenario without any VGs

Performance evaluation of louvered fin compact heat exchangers with vortex generators

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

Vortex generator arrays in air-cooled condensers for power plant applications

This study presents a numerical analysis of laminar convective heat transfer in a rectangular channel with different configurations of delta-winglet vortex generators (VG). A low and high Reynolds number case of 225 and 1123 is simulated which is representative of the lower and upper limits of the air velocity range between 1-5 m/s in a typical air-cooled condenser (ACC) system. The effects of interacting and non-interacting vortices on the Nusselt number and pressure drop are observed for the two Reynolds numbers. The low-Re cases flow cleaner through the channel, pressure drop is slightly lower for the array in comparison to a straight-line formation for two VG pairs; however, the increase in Nusselt number is higher in the straight-line configuration. For the high-Re cases the vortices generated are stronger and persist longer and interactions between vortices in the array configuration lead to Nusselt number increases, but a higher pressure drop results. The straight-line configuration has an almost equal increase in Nusselt number and pressure drop. Recommendations are made to further explore the VG placement using different Reynolds numbers for constructive augmentation of the vortices as the vortex behavior and interactions are mainly a affected by the inlet flow velocity

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with delta winglet-type vortex generators

Heat transfer rate, pressure loss and efficiency are considered as the most important parameters in designing compact heat exchangers. Despite different types of heat exchangers, fin-and-tube compact heat exchangers are still common device in different industries due to the diversity of usage and the low space installation need. The efficiency of the compact heat exchanger can be increased by introducing the fins and increasing the heat transfer rate between the surface and the surroundings. Numerous modifications can be applied to the fin surface to increase heat transfer. Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks, but they have drawbacks as well. They increase the pressure loss and this should be considered. In this paper, the thermal efficiency of compact heat exchanger with VGs is investigated in different variations. The angle of attack, the length and horizontal and vertical position of winglet are the main parameters to consider. Numerical analyses are carried out to examine finned tube heat exchanger with winglets at the fin surface in a relatively low Reynolds number flow for the inline tube arrangements. The results showed that the length of the winglet significantly affects the improvement of heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the results show that the optimization cannot be performed for one criterion only. More parameters should be considered at the same time to run the process properly and improve the heat exchanger efficiency

Improving Flat Plate Heat Transfer Using Flexible Rectangular Strips

Many engineering systems involve proper transfer of heat to operate. As such, augmenting the heat transfer rate can lead to performance improvement of systems such as heat exchangers and solar photovoltaics panels. Among the many existing and studied heat transfer enhancement techniques, a well-designed passive turbulence generator is a simple and potent approach to augmenting convective heat transfer. Two of the most recognized passive convective heat enhancers are wings and winglets. Their potency is attributed to the long-lasting induced longitudinal vortices which are effective in scooping and mixing hot and cold fluids. Somewhat less studied are flexible turbulence generators, which could further the heat transfer enhancement compared to their rigid counterpart. In the current study, the flexible rectangular strips are proposed, marrying the long-lasting vortex streets with the periodic oscillation, to maximize heat convection. This study was conducted in a closed-looped wind tunnel with 76 cm square cross-section. The effects of flexible strips on the turbulent flow characteristics and the resulting convective heat transfer enhancement from a heated flat surface are detailed in four papers which are presented as Chapters 3, 4, 5 and 6. In Chapter 3, the effect of the thickness of the strip is detailed. The 12.7 mm wide and 38.1 mm tall rectangular strip was cut from an aluminum sheet with thickness of 0.1, 0.2 and 0.25 mm. The incoming wind velocity was maintained at around 10 m/s, giving a Reynolds number based on the strip width of 8500. It is observed that the thinnest 0.1 mm strip could induce a larger downwash velocity and a stronger Strouhal fluctuation at 3H (strip height) downstream, leading to a better heat transfer enhancement. The peak of the normalized Nusselt number (Nu/Nu0) at 3H downstream of the 0.1 mm strip was around 1.67, approximately 0.1 larger than that of the 0.25 mm strip. In Chapter 4, the height effect of the strip is disclosed. The strip was 12.7 mm wide and 0.1 mm thick, with a height of 25.4 mm, 38.1 mm and 50.8 mm. The Reynolds number in this chapter was also fixed at around 8500, based on the strip width and the freestream velocity. It was found that the shortest, 25.4 mm strip could induce the closest-to-wall swirling vortices, and the largest near-surface downwash velocity toward the heated surface. Thus, the largest heat transfer augmentation was observed. At 9W (strip width) downstream, the 25.4 mm-strip provided the Nu/Nu0 peak of around 1.76, 0.26 larger than that associated with the tallest, 50.8 mm-strip. In Chapter 5, the effect of the transversal space of a pair of strips is expounded. A pair of 0.1 mm thick, 12.7 mm wide, and 25.4 mm tall aluminum rectangular flexible strips was placed side-by-side with a spacing of 1W (strip width), 2W and 3W. The Reynolds number based on the strip width was around 8500. The results showed that the 1W-spaced strip pair induced the strongest vortex-vortex interaction, the largest downwash velocity, and the most intense turbulence fluctuation. These resulted in the most effective heat convection. At Y=0 (middle of the strip pair) and X=9W, the largest Nu/Nu0 value of around 1.50 was identified when using the 1W-spaced strip pair. This was approximately 0.24 and 0.33 larger than that of the 2W- and 3W-spaced strip pairs. Chapter 6 presents the effect of freestream turbulence on the flat plate heat convection enhancement with a 12.7 mm wide, 25.4 mm tall and 0.1 mm thick flexible strip. A 6 mm thick sharp-edged orificed perforated plate (OPP) with holes of 38.1 mm diameter (D) was placed at 10D, 13D and 16D upstream of the strip to generate the desirable levels of freestream turbulence. The corresponding streamwise freestream turbulence intensity at the strip was around 11%, 9% and 7%. The Reynolds number based on the strip width and freestream velocity was approximately 6000. The freestream turbulence was found to diminish the effect of flexible strip in terms of the relative heat transfer enhancement (Nu/Nu0). This is due to the significant increase of Nu0 with the increasing freestream turbulence. In other words, the flexible strip could always improve the heat transfer, and the relative improvement is greatest for the largely laminar freestream case in the absence of the OPP. Chapter 7 summarizes the effect of all the parameters in previous chapters on the convective heat transfer enhancement. The results show that the freestream turbulence intensity (Tu) had the most significant effect in augmenting the averaged Nu/Nu0, and the local Nu/Nu0 correlated best with the local ke. The maximal averaged Nu/Nu0 over 23W downstream, within ±1 and ±4 strip widths cross-stream was found for Tu=7% case and Tu=11% case, respectively. Conclusions are drawn and recommendations are provided in Chapter 8

Improving Vortex Generators to Enhance the Performance of Air-Cooled Condensers in a Geothermal Power Plant

This report summarizes work at the Idaho National Laboratory to develop strategies to enhance air-side heat transfer in geothermal air-cooled condensers such that it should not significantly increase pressure drop and parasitic fan pumping power. The work was sponsored by the U.S. Department of Energy, NEDO (New Energy and Industrial Technology Development Organization) of Japan, Yokohama National University, and the Indian Institute of Technology, Kanpur, India. A combined experimental and numerical investigation was performed to investigate heat transfer enhancement techniques that may be applicable to largescale air-cooled condensers such as those used in geothermal power applications. A transient heat transfer visualization and measurement technique was employed in order to obtain detailed distributions of local heat transfer coefficients on model fin surfaces. Pressure drop measurements were obtained for a variety of tube and winglet configurations using a single-channel flow apparatus that included four tube rows in a staggered array. Heat transfer and pressure drop measurements were also acquired in a separate multiple-tube row apparatus in the Single Blow Test Facility. In addition, a numerical modeling technique was developed to predict local and average heat transfer for these low-Reynolds number flows, with and without winglets. Representative experimental and numerical results were obtained that reveal quantitative details of local finsurface heat transfer in the vicinity of a circular tube with a single delta winglet pair downstream of the cylinder. Heat transfer and pressure-drop results were obtained for flow Reynolds numbers based on channel height and mean flow velocity ranging from 700 to 6500. The winglets were of triangular (delta) shape with a 1:2 or 1:3 height/length aspect ratio and a height equal to 90% of the channel height. Overall mean fin-surface heat transfer results indicate a significant level of heat transfer enhancement (in terms of Colburn j-factor) associated with deployment of the winglets with circular as well as oval tubes. In general, toe-in (common flow up) type winglets appear to have better performance than the toe-out (common flow down) type winglets. Comparisons of heat transfer and pressure drop results for the elliptical tube versus a circular tube with and without winglets are provided. During the course of their independent research, all of the researchers have established that about 10 to 30% enhancement in Colburn j-factor is expected. However, actual increase in heat transfer rate from a heat exchanger employing finned tubes with winglets may be smaller, perhaps on the order of 2 to 5%. It is also concluded that for any specific application, more full-size experimentation is needed to optimize the winglet design for a specific heat exchanger application. If in place of a circular tube, an oval tube can be economically used in a bundle, it is expected that the pressure drop across the tube bundle with the application of vortex generators (winglets) will be similar to that in a conventional circular tube bundle. It is hoped that the results of this research will demonstrate the benefits of applying vortex generators (winglets) on the fins to improve the heat transfer from the air-side of the tube bundle