Air-side Heat Transfer Coefficient Research Articles

Heat transfer and power consumption of an optimized low-pressure compact drizzling cooling module integrated PEMFC stack radiator

The application of nozzle-assisted spray cooling to proton exchange membrane fuel cell (PEMFC) stack radiators has shown superior performance when compared with conventional air-cooled radiators. However, challenges remain with regard to high power consumption. This study experimentally optimized a low-pressure compact drizzling module coupled with a stack radiator as an alternative spray cooling strategy. The effects of the distance between the drizzling module and the radiator (30–90 mm) were examined, along with the influence of the spray flow rate/temperature, air velocity/temperature, and coolant flow rate on the thermal performance. The results showed that among the operating variables, the impact of the drizzling distance decreased with an increase in air velocity. Moreover, the drizzling performance was significantly influenced by the spray flow rate and air velocity, with minimal impact from the coolant-side variables. Optimal thermal performance was achieved at a distance of 50 mm and spray flow rate of 0.4 LPM, resulting in a 142 % improvement in heat rejection compared to an air-cooled radiator and a 132 % increase in the air-side pressure drop. Additionally, heat rejection only decreased by 7.3 % as the spray temperature increased from 40 °C to 70 °C. Two novel empirical correlations were developed, predicting heat transfer improvement and air-side heat transfer coefficient with over 85 % accuracy. These findings underscore the potential of drizzling cooling for stack radiators in fuel-cell vehicles, while the empirical correlations offer valuable insights for the development of spray-cooled radiators for next-generation electric vehicles.

CFD Air Flow Evaluation of Finned Tube Evaporator for Refrigerated Display Cabinet Application

This study is aimed to develop a simulation to improve the performance of the finned tube evaporator which is applied to the refrigerated display cabinet. CFD model was developed to be able to analyse the characteristics of air flow inside the fin gap and air side heat transfer coefficient. Geometry of the model of overall finned tube evaporator is considered covering two aluminium wavy fins with an air flow in between, combination of staggered cooper tubes with refrigerant flow inside. Fin gap is designed 4 mm to anticipate frost on the fin surface that can block air flow. Turbulence models used in the study is the realizable k-ε turbulence which had the best performance turbulence model and it was validated with secondary data from previous studies and shows the lowest error only 5.9 %. The use of CFD was found to be sufficiently representative of the heat transfer characteristics of evaporators, and acted as an effective simulation tool to determine the heat transfer coefficient in order to improve efficiency in terms of improved design. The characteristics of air flow between the fin gap and around the tube was obtained various and complex. In the case study the entry velocity of 1.7 m /s at the highest turbulence condition of the first row can reach speeds of 2.75 m/s. Hight turbulence regime in flow can indicate higher the heat transfer coefficient of the evaporator.

Frosting characteristics of microchannel heat exchangers: Parametric studies and correlation development

Recently, microchannel heat exchangers (MCHX) have been widely used in heat pump systems due to their high surface-to-volume ratio and high efficiency of heat transfer. However, when the MCHX operates in frosting conditions, its performance is adversely affected. To study the frost formation and distribution characteristics of MCHX, a comprehensive parametric study was performed for the MCHX with a larger fin pitch. The effects of fin geometries and environmental parameters on the frosting characteristics of MCHX were studied. Fin height (Fh) can effectively inhibit frost formation, which is due to the increase of surface temperature with the increase of Fh. When Fh increased from 6.0 mm to 11.8 mm, the frost time increased by 30.5 %. However, the increase in Fh also brings the penalty of reduced heat transfer rate. The effect of fin depth (Fd) on frost formation performance is small because the frost is mainly deposited in the front part, resulting in the rear fin cannot work efficiently. Fin pitch (Fp) is the most significant structural parameter affecting frosting performance. When Fp increases from 1.6 mm to 4 mm, the frosting time increases by 4 times and frost mass increased by 2.2 times. This is because with the increase of Fp, the frost blocking in the front part can be effectively delayed, and the distribution uniformity of the frost layer is improved. Lower surface temperature and higher humidity ratio significantly promote frost formation and reduce the uniformity of frost distribution. When the inlet temperature is reduced from −5 °C to −8 °C, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 55.7 %. When the relative humidity increases from 76 % to 92 %, the frosting time is shortened by 2.15 times, and the frost mass is reduced by 42.5 %. The air velocity has a minor impact on the growth rate of frost in the front part. However, as the air velocity increases, the uniformity of frost distribution is significantly improved. When the air velocity increased from 1.1 m s−1 to 2.2 m s−1, the mass of accumulated frost increased by 29.5 %. Furthermore, frost thickness and air-side heat transfer coefficient correlations were developed by testing 12 samples. The developed heat transfer coefficient correlation can accurately capture the changing trend of the j-factor. This study provides a fundamental understanding of the frosting characteristics of MCHX and can effectively guide the louvered fin optimization.

Simulation on the performance and cooling capacity compensation solution of a loop thermosyphon system under fan failure conditions

Fan failure of loop thermosyphon systems (LTS) is a major factor affecting thermal safety of data centers. This paper develops a distributed-parameter model to accurately predict the overall and local heat transfer performance of a loop thermosyphon (the terminal equipment of LTS) under fan failure conditions. The simulation results show that the positions of faulty fans have a significant influence on the evaporator's local airside heat transfer coefficient, but have slight influences on the evaporator's local refrigerant heat transfer coefficient and the condenser's overall heat transfer coefficient, which would not increase the risk of heat transfer performance. Thus, fan airflow rate backup can be used to handle fan failure. Then, the effect of operating parameters including backup airflow rate (Bfan), cold water temperature (Tcw) and increment of cold water flow rate (Im,cw) on the effectiveness and efficiency of the cooling capacity compensation under fan failure conditions is analyzed by using the Taguchi method with an L16 orthogonal array. The Bfan is the primary factor contributing 81.51% on cooling capacity compensation, with Tcw and Im,cw contributing 15.25% and 2.38%, respectively. Considering the energy efficiency, the order of adjustment should be Bfan first, Im,cw second, and Tcw last when fan failure occurs.

Numerical study of heat transfer and fluid flow in the offset strip-fin heat exchanger: A fin-by-fin analysis

Heat transfer and fluid flow in the offset strip-fin heat exchanger are studied numerically, fin by fin. The impact of the offset strip-fin geometry on j/f performance is also evaluated. The mathematical model is validated against j- and f-factor correlations from the literature. The numerical analysis revealed that the offset strip-fin heat exchanger achieves best j/f performance in the laminar flow region (Redh < 700). At higher Reynolds numbers, the j/f factor deteriorates under the impact of flow recirculations, oscillatory wakes and the contribution of form drag. Compared to the continuous rectangular fin, the offset strip-fin increases the air-side heat transfer coefficient by 50% at Redh = 100 and by 120% at Redh = 4000 while the pressure drop increases by a factor of 3–5. Generally, the offset strip-fin achieves j/f performance between 0.20 and 0.30, with maximum values found in the laminar flow region (Redh 〈1000). Offset strip-fin geometries with reduced fin space and increased wall thickness strongly affect the form drag, which reduces the j/f performance, especially for Redh > 1000. The fin height weakly influences the performance, and the j/f factor is between 0.22 and 0.23 for a wide range of fin heights.

Influence of vibration parameters and fin structure parameters on heat transfer performance under vibration conditions

The occurrence of vibration in engineering vehicles during operation is a common phenomenon, and the utilization of vibration-enhanced heat transfer serves as a viable approach to enhance the efficiency of heat transfer in radiators. This study designs a vibration test platform for heat exchangers to investigate the impact of different vibration parameters on heat transfer performance. Additionally, a fin vibration model is constructed using overlapping grid technology to simulate the behavior of flat fins under varying vibration conditions. Corrugated fins were chosen for investigation under vibration conditions, utilizing an orthogonal experimental approach to analyze the impact of fin structural parameters on heat exchanger performance. Findings indicate that vibration enhances the air side heat transfer coefficient of the heat exchanger, in experimental settings, the gas side heat transfer coefficient of the heat exchanger demonstrated a maximum increase of 12.28%. Simultaneously, vibration induces an escalation in pressure loss, with the maximum drop reaching 28.72%. The heat transfer factor j of the heat exchanger exhibits an augmentation in response to heightened vibration intensity, while they diminish with escalating speed. Furthermore, vibration alters the impact of structural factors of corrugated fins on heat transfer efficacy. During vibration, the sensitivity of fin height and wavelength intensifies, whereas the sensitivity of pitch and wave amplitude diminishes. The findings of this research offer valuable insights for further investigation into the heat transfer capabilities of fins under vibration.

Superhydrophobic Microchannel Heat Exchanger for Electric Vehicle Heat Pump Performance Enhancement

Battery-powered electric vehicles (EVs) have emerged as an environmentally friendly and efficient alternative to traditional internal combustion engine vehicles, while their single-charge driving distances under cold conditions are significantly limited due to the high energy consumption of their heating systems. Heat pumps can provide an effective heating solution for EVs, but their coefficient of performance (COP) is hampered by heat transfer deterioration due to frost accumulation. This study proposes a solution to this issue by introducing a microchannel heat exchanger (MHE) with superhydrophobic surface treatment (SHST) as a heat pump evaporator. A computational fluid dynamics MHE model and a dynamic heat pump model are developed and rigorously validated to examine the detrimental impact of frost accumulation on heat transfer, airflow resistance, and heat pump performance. When the frost layer thickness is 0.8 mm at a given air-side velocity of 1.0 m/s, the air-side heat transfer coefficient can be reduced by about 75%, and the air-side pressure drop sharply increases by 28.4 times. As frost thickness increases from 0 to 0.8 mm, the heating capacity drops from 3.97 to 1.82 kW, and the system COP declines from 3.17 to 2.30. Experimental results show that the frost thickness of the MHE with SHST reaches approximately 0.4 mm after 30 min, compared to that of 0.8 mm of the MHE without SHST, illustrating the defrosting capability of the superhydrophobic coating. The study concludes by comparing the performance of various heating methods in EVs to highlight the advantages of SHST technology. As compared to traditional heat pumps, the heating power consumption of the proposed system is reduced by 48.7% due to the defrosting effect of the SHST. Moreover, the single-charge driving distance is extended to 327.27 km, an improvement of 8.99% over the heat pump without SHST.

Modeling and Optimization of a Micro-Channel Gas Cooler for a Transcritical CO2 Mobile Air-Conditioning System

This study focuses on developing and optimizing of a microchannel gas cooler model for evaluating the performance of a transcritical CO2 mobile air-conditioning system. A simulation model is developed with the aid of MATLAB R2022a. A segment-by-segment modeling approach is utilized by applying the effectiveness-NTU method. State-of-the-art heat transfer and pressure drop correlations are used to obtain air and refrigerant side heat transfer coefficients and friction factors. The developed model is validated through a wide range of available experimental data and is able to predict a gas cooler capacity and pressure drop within an acceptable range of accuracy. The average errors for a gas cooler capacity and pressure drop are 3.79% and 10.24%, respectively. Furthermore, a parametric optimization method is applied to obtain optimal microchannel heat exchanger dimensions, including the number of tubes, microchannel ports, and passes. Different combinations were selected within the practical range to obtain optimal dimensions while keeping the total core volume constant. The simultaneous effect of the number of tubes, the number of ports in each tube, and the number of passes is determined. The objective of the current optimization technique is to minimize the pressure drop for the specific design capacity under different operating conditions without changing the overall volume of the gas cooler. The average pressure drop reduction for the optimal geometry as compared with the baseline geometry under all operating conditions is about 15%. The results from this study can be used to select an optimal geometric design for the required design capacity with a minimal pressure drop without the need for expensive prototype development and testing.

Numerical investigation on Enhancing Heating performance in Automotive Radiator

In this paper, the improvement of heat exchange in a compact heat exchanger that is used in the automotive cooling system is numerically studied due to the importance that this part of the automotive represents in removing the heat generated in the automotive engine. The study adopted CFD simulation to introduce a new design of radiator that included changing the geometry and material of the tube while maintaining the same cross-section area In addition, the study also included replacing the shape of the fins from the louver to plain fin with the increase in the number of tubes. The study used a water-Ethylene glycol mixture (50:50) as a hot fluid with four flow rates (10, 12, 18, 24) l/h, (75, 85, 95) °C inlet temperature, and air as a cold fluid with 1.5, 2.5, 4.5, 5.5) m/s and (35) °C inlet temperature. Through the results obtained, the designed model appeared to have a higher thermal performance than the standard model due to the material and shape of the tube and fins. The designed model performed well, achieving an improvement ratio of (18.6%) for the heat transfer rate, (13.8%) for the overall heat transfer coefficient for the hot fluid, and (29.8%) for the air-side heat transfer coefficient

Mathematical Model of Air Dryer Heat Pump Exchangers

This paper presents a mathematical model of heat pump exchangers and their thermal interaction with a fan for an air dryer. The calculation algorithm developed for the finned heat exchangers is based on the ε-NTU method, allowing the determination of air side and refrigerant side heat transfer coefficients, evaporator and condenser heat capacity and air parameters at the dehumidifier outlet with known exchanger geometries, initial air parameters and mass flow rate. The model was verified on an industrial dehumidifier test bench. This enabled the heat transfer coefficients for the exchanger to be calculated as a function of the speed and, therefore, the power of the fan’s drive motor. An increase in fan performance on the one hand results in an increase in the heat transfer rate, but, on the other hand, it causes an increase in the total energy consumption of the motor. Thus, while it causes an increase in drying capacity, it also causes an increase in the energy consumption of the dehumidifier. In order to optimise the unit in terms of energy consumption, it is therefore necessary to determine a function that relates the amount of heat exchanged to the efficiency of the fan.

Effect of the segmented fin height on the air-side performance of serrated welded spiral fin-and-tube heat exchangers

Serrated welded spiral fin-and-tube heat exchangers (SWSFTHXs) are experimentally investigated. The work primarily focuses on the effects of the segmented fin height ( h s ) with various fin pitches ( f p ) on the air-side performance (ASP). Experimental results show that SWSFTHXs provide a higher air-side heat transfer coefficient than the plain welded spiral fin-and-tube heat exchangers (PWSFTHXs) at the same f p . The h s has a significant effect on the Nusselt number ( Nu ) and Colburn factor ( j ), whereas f p clearly has a greater effect on the friction factor ( f ) and Euler number ( Eu ) than h s . Furthermore, the Nu , j , f , and Eu correlations for PWSFTHXs and SWSFTHXs are also proposed.

Numerical Study of Fin-and-Tube Heat Exchanger in Low-Pressure Environment: Air-Side Heat Transfer and Frictional Performance, Entropy Generation Analysis, and Model Development.

Heat transfer and frictional performance at the air-side is predominant for the application and optimization of finned tube heat exchangers. For aerospace engineering, the heat exchanger operates under negative pressure, whereas the general prediction models of convective heat transfer coefficient and pressure penalty for this scenario are rarely reported. In the current study, a numerical model is developed to determine the air-side heat transfer and frictional performance. The influence of air pressure (absolute pressure) is discussed in detail, and the entropy generation considering the effect of heat transfer and pressure drop are analyzed. Furthermore, prediction models of air-side thermal and frictional factors are also developed. The results indicate that both the convective heat transfer coefficient and pressure penalty decrease significantly with decreasing air pressure, and the air-side heat transfer coefficient is decreased by 64.6~73.3% at an air pressure of 25 kPa compared with normal environment pressure. The entropy generation by temperature difference accounts for the highest proportion of the total entropy generation. The prediction correlations of Colburn j-factor and friction factor f show satisfactory accuracy with the absolute mean deviations of 7.48% and 9.42%, respectively. This study can provide a reference for the practical application of fined tube heat exchangers under a negative pressure environment.

An innovative clustering technique to generate hybrid modeling of cooling coils for energy analysis: A case study for control performance in HVAC systems

Despite past studies, no comprehensive models or empirical correlations cover all aspects of performances of cooling coils under different flow regimes (laminar, transition, and turbulent). Moreover, the cooling coil is characterized by a highly nonlinear dynamic subject to multiple inputs, coupling between the latent and sensible heat transfer modes, uncertain disturbances, and strong dependence of the overall heat transfer coefficient on the flow type, all causing significant challenges when it comes to modeling. Therefore, a hybrid layer structure model was adopted in this study to overcome these challenges. The new approach used two different optimization methods, Neural Networks' Weights and Takagi-Sugeno (TS) fuzzy, and the hybrid layers tuned by the Gauss-Newton algorithm (GNA). The proposed model covered three types of fluid flow to represent the dynamic behavior of the water-side and air-side heat transfer coefficients, each of which was divided into seven clusters and had its unique TS consequence. This study also administered meaningful fitness tests in the responses of the eleven independent variables that serve as its inputs. Furthermore, its application shows the control performance saving more than 44% of HVAC system energy. Based on the results, it was concluded that the proposed model is suitable for estimating energy and cost savings for electric power and water flow rate efficiency. In addition, the response of all types of output flow can be evaluated when changing eleven independent variables that are manipulated by three different controllers.

Comparison of Local and Averaged Air-Side Heat Transfer Coefficients on Fin-and-Tube Heat Exchangers Obtained With Experimental and Numerical Methods

Abstract Numerical methods are often used to obtain two-dimensional air-side heat transfer coefficients (HTCs) on heat exchanger (HX) fin surfaces. The model's accuracy is usually verified through averaged HTCs by comparing with published experimental results. However, substantial disagreement is not uncommon and can hardly be explained by averaged HTCs. This study focuses on comparing experimental, local air-side HTCs to numerical ansys fluent results. A mass transfer experimental method was employed to obtain HTC distributions on the fin-and-tube HX fin surfaces. Therefore, disagreements between the experimental and numerical results can be explained in detail. There are several significant findings: inaccurate predictions of local HTCs are observed even though the averaged HTCs from the numerical method may agree with the averaged experimental results under some conditions. The models fail to capture horseshoe vortices, underestimate the HTCs in the wake region of the tubes, and overpredict row-by-row HTC degradation. Moreover, the accuracy of the numerical model decreases when the complexity of geometry increases. For the flat plate, numerically obtained HTCs agree with the experimental results within 10%. However, the error is more than 30% for the eight-row HX. Nonetheless, the model's accuracy becomes worse at higher airflow velocities. Oversized fin-and-tube HXs with multiple tube rows are often selected as a result of using the underpredicted averaged air-side HTCs from the numerical computational fluid dynamics (CFD) simulations. Thence, the authors have proposed a corrective method to improve the accuracy of the numerical model.

Analysis of external heat dissipation enhancement of oil-immersed transformer based on falling film measure

With the increase of power load, the overheating problem of oil-immersed transformer in operation cannot be ignored. Therefore, it is necessary to propose reasonable heat dissipation enhancement measure for oil-immersed transformer in operation and study the influence of the measure on the internal temperature of transformer. The heat transfer process of oil-immersed transformer is analyzed, and the air-side heat transfer coefficient is pointed out to be the important parameter limiting the heat dissipation capacity of the transformer. In order to improve heat dissipation capacity, the measures based on cooling fans and falling film are proposed and the calculation methods for the air-side heat transfer coefficient are conducted, respectively. Moreover, the top-oil temperature and hot-spot temperature under two measures are compared by the thermal-fluid coupling model of oil-immersed transformer. For the transformer operating at rated load and the ambient temperature of 25?C, after the cooling fans are adopted, the top-oil temperature and hot-spot temperature can be reduced by 22.4?C and 20.7?C, while after the falling film is adopted, the top-oil temperature and hot-spot temperature can be reduced by 31.2?C and 28.7?C, respectively. The results show that the falling film is a more effective heat dissipation enhancement measure, which can meet the higher load demand of oil-immersed transformer.

Novel structural designs of fin-tube heat exchanger for PEMFC systems based on wavy-louvered fin and vortex generator by a 3D model in OpenFOAM

Heat management is crucial to the stable and high-efficiency operation of proton exchange membrane fuel cell (PEMFC) system. However, fin-tube heat exchangers (FTHE) of traditional internal combustion engine vehicles require further optimizations to be applicable to PEMFC vehicles. In the paper, a three-dimensional steady-state radiator model is developed in OpenFOAM to investigate three novel structural designs based on wavy-louvred (WL) fin and vortex generators (VGs). The established model has been carefully validated against experimental data and correlation reference. To comprehensively evaluate radiator performances, the air side heat transfer coefficient, pressure drop, outlet air temperature, heat flux, and JF factor are adopted. It is found that the FTHE with L-VGs has the highest heat transfer coefficient while the FTHE with WL-VGs has the highest pressure drop. The temperature, velocity, and pressure distribution are further demonstrated to reveal performance enhancement mechanisms. It is seen that the heat exchangers with additional VGs produce two sections of high-temperature wakes near the wall, which not only promotes the heat convection but also contributes to the heat exchange in the nearby area. Meanwhile, a low-speed vortex zone behind VGs appears and generates longitude vortex, making the air stream stay longer at fin surfaces. The air flow in FTHE with WL is not as much separated as the conventional FTHE since the zigzag wavy louver restricts flow separation. The paper gives valuable suggestions for cooling capability improvement and radiator volume diminution.

Experimental and Mathematical Research of Convection Heat Transfer for a High Integral Finned Tube Heat Exchanger

This research exhibits an experimental and numerical investigation for the characteristics of heat transfer by utilizing an integral and smooth high finned tube and shows how the integral high fins affect in improving the transfer of heat. In the experimental work, the experimental rig consists of a cold water loop, a hot air loop and the trial part which is a concentric parallel flow double pipe heat exchanger. The first test smooth tube made of brass has external and internal diameter (32 mm) and (22 mm), respectively, the second test section made of the same material has external fins, the third test section possesses internal fins, and the fourth test section has internal and external fins. The dimensions of the fin are (2 mm) in thickness, (2 mm) in height and pitch (2.5 mm) center to center. And, the water flow rates are (6, 8, 10, 12 and 14 L/min). The inlet water to the trial tube was at temperatures (15, 25, 35, and 45oC). The investigational results manifested that the air side heat transfer coefficient of the smooth tube was lesser than the integral high finned tube. The ratio of improvement when using the integral high finned tube was (47.5%, 60.5% and 67.5 %). Numerical simulation was applied on the current heat exchanger to study both the transfer of heat and the field of flow by using ANSYS, FLUENT15 package. Steady state, Newtonian flow, incompressible and three dimensional analyses were assumed. The comparison between experimental work and numerical results elucidated a good agreement.

Using patterned surface wettability to enhance air-side heat transfer through frozen water droplet vortex generators – Part II: CFD simulation results

• Frozen water droplets can serve as vortex generators (VGs) to increase air-side heat transfer. • Hemispherical domes (i.e. model droplets) were simulated numerically to quantify performance. • Ten different configurations were investigated with the staggered VG pattern performing better than the inline pattern. • Changing the longitudinal spacing between VGs led to better performance. • Heat transfer enhancements from 14.0 to 75.9% were observed with pressure drop penalties from 35.7 to 165.6%. In this study, a novel technique for deploying hemispherical vortex generators (VGs) aimed at heat transfer enhancement via the naturally-occurring processes of condensation and freezing was proposed and investigated. By using patterned surface wettability to collect condensate and encourage coalescence in predetermined locations, it was hypothesized that large frozen droplets could be formed in various configurations which could serve as VGs. This approach was simulated numerically in the present communication (Part II) and examined experimentally in a previous study (Part I). For the CFD simulations, ten different configurations in channel flow emulating a fin density of 5 FPI were investigated. Inlet air velocities of 1.0 and 2.0 m s −1 at 20°C were examined for the flow through the channel where the walls and VGs were set to -9°C to match the experimental conditions used in Part I. The air-side heat transfer coefficient, pressure drop, and temperature changes were then calculated after a converged solution was reached. Compared to a baseline configuration without VGs, heat transfer enhancements ranging from 14.0 to 75.9% were observed, with corresponding pressure drop increases (or penalties) ranging from 35.7 to 165.6%.

A comparison study on thermal performance enhancement of corrugated oil cooler with internal turbulators – Numerical and experimental approach

Oil coolers play an important role in dissipating excess heat from engine oil to surroundings. In this work, performance study of an oil cooler with internal turbulators of dimple and offset type is performed using numerical approach and compared with experimental results for validation. In numerical approach, oil cooler with turbulators are numerically modeled using Conjugate Heat Transfer (CHT) approach to solve for heat transfer characteristics. Turbulence physics modeling is done using Reynolds Averaged Navier Stokes equations (RANS) through realizable k-epsilon model with standard wall function. The air side Heat Transfer Coefficient (HTC) estimation is done with CHT model using one corrugation, as the air flow in fins is periodic. In experimental study, Required Inlet Temperature Difference (ITD), Oil Flow Rate, Air Flow Rate is maintained, measurements are recorded to create performance data matrix with combinations of various operating data points for air and oil sides respectively. Numerical results are compared with experimental results obtained; Deviation for heat rejection of oil-cooler with dimple turbulator fall in range of ±7%, offset turbulator fall in the range of −2.0 to 9.4%. Numerical data shows that the thermal performance of offset type is better than dimple type by 10–13.5%, while the test data depicts that performance of oil-cooler with offset turbulator is high by 12–19% than dimple type and thereby proving its performance superiority. In offset turbulator additional turbulent scales of the offset length scale range are generated, these additional scales are evident by increasing the Turbulent Kinetic Energy (TKE) of 46–51% than dimple turbulator from numerical study. Thus, for same plate and fin, oil-cooler with offset turbulator has better thermal performance than oil cooler with dimple turbulator.

Thermal and hydraulic characteristics of multiple row spiral finned tube heat exchangers

An experimental investigation for the thermal and hydraulic characteristics of a spiral finned-tube heat exchanger was performed to understand its cooling performance for a developed novel defrosting technique. Two identical spiral finned-tube heat exchangers were manufactured; one having four rows and other having five rows. Both heat exchangers tubes were arranged inline. The heat exchangers had a tube diameter of 9.36 mm, spiral fins of 23.4 mm outer diameter, and 3.6 mm fin pitch (277 fins/m). The overall heat transfer coefficient was calculated using the measured temperatures and flow rates of each stream, and utilizing the logarithmic mean temperature difference method. The air-side heat transfer coefficient was determined using the calculated overall heat transfer coefficient and the tube-side heat transfer coefficient. The air-side pressure drop across the heat exchanger was also measured using a differential pressure digital manometer. The experimental data covered Reynolds number range of 1500 to 6000 and a Prandtl number of 0.7. Empirical correlations of Nusselt number, Colburn factor and friction factor versus Reynolds number were developed using least squares regression. The results showed that there is no significant variation in air-side heat transfer coefficient for the four and five rows spiral finned-tube heat exchanger and their Colburn factor is almost constant over the tested range of Reynolds number.