Air-side Convective Heat Transfer Research Articles

The heat transfer potential of compressor vanes on a hydrogen fueled turbofan engine

Hydrogen is a promising fuel for future aviation due to its CO2-free combustion. In addition, its excellent cooling properties as it is heated from cryogenic conditions to the appropriate combustion temperatures provides a multitude of opportunities. This paper investigates the heat transfer potential of stator surfaces in a modern high-speed low-pressure compressor by incorporating cooling channels within the stator vane surfaces, where hydrogen is allowed to flow and cool the engine core air. Computational Fluid Dynamics simulations were carried out to assess the aerothermal performance of this cooled compressor and were compared to heat transfer correlations. A core air temperature drop of 9.5 K was observed for this cooling channel design while being relatively insensitive to the thermal conductivity of the vane and cooling channel wall thickness. The thermal resistance was dominated by the air-side convective heat transfer, and more surface area on the air-side would therefore be required in order to increase overall heat flow. While good agreement with established heat transfer correlations was found for both turbulent and transitional flow, the correlation for the transitional case yielded decent accuracy only as long as the flow remains attached, and while transition was dominated by the bypass mode. A system level analysis, indicated a limited but favorable impact at engine performance level, amounting to a specific fuel consumption improvement of up to 0.8 % in cruise and an estimated reduction of 3.6 % in cruise NOx. The results clearly show that, although it is possible to achieve high heat transfer rate per unit area in compressor vanes, the impact on cycle performance is constrained by the limited available wetted area in the low-pressure compressor.

Analysis of transport phenomena on cooling a flat surface using swirl jet impingement with crossflow

Enhancing airside convective heat transfer is a challenging problem due to the poor thermal properties of air and the high thermal resistance of the thermal boundary layer. This study presents an innovative method to improve the convective heat transfer of cross airflow over a flat plate using a Swirl jet to suppress the boundary layer thus reducing the thermal resistance. A three-dimensional numerical analysis was performed to study the transport phenomena that occur when a swirl air jet is issued perpendicular to a crossflow of air flowing over a hot flat plate. The finite element method with SST k-ω turbulence model was employed for this numerical study after validation with experimental data from the literature. A parametric study was performed by varying Reynolds number of the jet, Rej(2500−10000), dimensionless jet to plate spacings, Z/D(1−2), swirl number, SW(0.3−0.9), and blockage ratios, BR(0.4−0.6). The thermal performance was assessed by the average heat transfer coefficient, h‾j, in addition to the ratio of heat transfer coefficient between the proposed setup involving crossflow with jet to the reference case of crossflow without jet, h‾j/h‾woj, thus, a correlation is obtained for the average heat transfer coefficient. The friction factor, f, and the pumping power were used to evaluate hydraulic performance. The performance evaluation coefficient, PEC, was chosen to identify the best possible thermal and hydraulic performance. Results show that for all the cases under study, there is a clear enhancement of the heat transfer coefficient when employing a combination of swirl jet with crossflow over a flat plate compared to the case of crossflow without jet. The maximum value of PEC was 1.8 occurring at Reynolds number, Rej, of 10000, swirl number of 0.9, blockage ratio of 0.6, and jet-to-plate spacing of 2D.

Numerical investigation and design optimization of a novel polymer heat exchanger with ogive sinusoidal wavy tube

In this paper, a novel polymer bare-tube heat exchanger (BTHX) with an ogive tube shape and a sinusoidal wavy channel is proposed and investigated for the liquid-to-gas application. The thermal-hydraulic performance of the proposed polymer BTHX is calculated using a validated computational fluid dynamics (CFD) simulation model, and the performance is optimized by setting nine independent design variables. A selective-series CFD method with an approximation-assisted optimization technique is developed for the multi-objective optimization to reduce the computational time. As a result, the thermal-hydraulic performance of the optimized novel polymer BTHX shows 91.5% and 134.9% of that of the target aluminum louvered-fin micro-channel heat exchanger (MCHX), respectively. This is mainly due to the improved air-side heat transfer coefficient of the proposed polymer BTHX by 70.2% compared to the target aluminum MCHX. Compared to the recent teardrop-shaped polymer BTHX, the thermal-hydraulic performance of the proposed BTHX is significantly improved. Regarding the polymer thermal conductivity, it is found that the thermal performance degradation can be minimized with at least 8.0 W·m−1·K−1 of the thermal conductivity. The flow field of the proposed polymer BTHX is also discussed and shows that the unique structure of the sinusoidal wavy tube can reduce the air-side pressure drop and promote the mixing of the air flow, thereby improving the air-side convective heat transfer.

Study on the air-side flow and heat transfer characteristics of corrugated fin under low-pressure environment

ABSTRACT Many troubles appear when vehicles work in the plateau region, and the main reason is that the heat-transfer capability of radiator in the vehicle cooling system reduces under the plateau low air pressure environment. Hence, the research on the flow and heat transfer characteristics of radiators in plateau low air pressure environment is necessary. In this paper, the experimental study on the thermal-hydraulic characteristics of corrugated fin radiator is conducted on a wind tunnel test-bed with low pressure function. The results illustrate that the air-side convective heat transfer coefficient of the corrugated fins decreases with the decrease of the ambient pressure, and the Colburn factor (j) and Friction factor (f) increase simultaneously. When the gauge pressure is −44 kPa, the air-side convective heat transfer coefficient is 34% lower than that under normal pressure on average, and the Colburn factor and Friction factor increase by an average of 21.1% and 55.6%, respectively. On this basis, the entransy dissipation and field synergy angle distribution of the corrugated fin radiator under low-pressure environment are studied. It is showed that the entransy dissipation number at −44 kPa gauge pressure reduces by 65% on average. The air-side field synergy angle increases with the decrease of the ambient pressure. Finally, the experimental correlations of the Colburn factor and Friction factor are performed, and the average deviations of the calculated results are 2.8% and 3.3%, respectively.

Research on the air-side thermal hydraulic performance of louvered fin and flat tube heat exchangers under low-pressure environment

ABSTRACTIn order to solve the problem of overheating of the engine and cooling system of the vehicle in the plateau low-pressure environment, it is necessary to study air-side thermal-hydraulic performance of the vehicle cooling system heat exchanger under low-pressure environment. In this paper, the experimental study on the louvered fin and flat tube heat exchanger of the vehicle is carried out on the low-pressure wind tunnel test bench. The results show that at a gauge pressure of −44 kPa, the air-side convective heat transfer coefficient is reduced by 33.5%~23.3% compared with 0 kPa; the Colburn j-factor is increased by 36.8% on average; the f factor is increased by 51.2% on average. Based on this, numerical simulations are carried out to study the heat transfer and flow characteristics of louvered fins with different structural parameters under low-pressure environment. The empirical correlations of Colburn j-factor and Friction factor f are fitted by multiple linear regression. In the range of gauge pressure from 0 kPa to −44 kPa, the average deviation of the correlation calculation result of Colburn j-factor is 3.4%, and the average deviation of the correlation calculation result of f factor is 5.5%. The research results can provide a basis for the design of louvered fin heat exchangers utilized in the plateau environment.

Experimental study on heat transfer and pressure drop characteristics of fin-and-tube surface with four convex-strips around each tube

In the present study, heat transfer and friction characteristics of heat transfer exchanger with a new type of enhanced fin – convex fin was experimentally investigated. For comparison, a plain-fin heat transfer surface with the same dimension was also tested. At low Reynolds number the airside convective heat transfer coefficient of the convex-fin is 25% higher than that of the plain-fin. At high Reynolds number the heat transfer coefficients of the convex-fin is slightly higher than the heat transfer coefficient of the plain fin. The pressure drop of the new enhanced fin increases by 16%. The convex-fin is suitable for enhancing heat transfer at low and middle velocity condition commonly encountered in air-cooling industry.

Effects of frost growth on louvered folded fins of microchannel heat exchangers on the time-dependent air side convective heat transfer coefficient

This paper investigated the frost formation on louvered folded fins in microchannel type heat exchangers and the effects of frost accumulation on the time dependent air side convective heat transfer coefficients during transient heat and mass transfer processes. Seven fin geometries, which were commonly used in outdoor microchannel type heat exchangers of air-source heat pump systems, were investigated for a wide range of temperatures and humidity promoting frost growth. The air-side heat transfer coefficient was depended on the rate of frost growth and it sometimes increased during early frost formation and then it decreased sharply. Frost thickness on the fin leading edge, air face velocity, and corresponding heat transfer coefficients and pressure drops were measured and semi-empirical correlations were developed. The frost thickness and air face velocity were key variables for estimating a local time-dependent air-side Reynolds number and for describing the transient behaviour of the local air-side heat transfer coefficient of the fins. The newly developed correlations identified the main parameters that describe frost growth and heat transfer on louvered folded fins under frosting operating conditions and they agree with the experimental data of the present work and from the literature.

Research on the Heat Transfer and Flow Characteristics of a New Type of Aluminum Noncontact Thermal Resistance Finned Tubes

With energy consumption increasing sharply and copper resource decreasing rapidly, the demand for a high-performance and efficient heat exchanger becomes more and more pressing. Therefore, it is very important to develop new technology, new materials, and low-cost and high-performance fin tubes, which is the main component of heat exchanger. In this paper, a new type of aluminum noncontact thermal resistance finned tube (ANCRF) is studied experimentally on the standard wind tunnel test bench. The performance curves of heat transfer and resistance and the experimental correlations were obtained in the experiment range of Reynolds number, which indicates that ANCRF has excellent heat transfer and flow resistance performance. Compared with a louvered fin tube and plate fin tube, the air-side convective heat transfer coefficient of ANCRF was approximately 63.06 and 176.33% higher on average; the friction factor was approximately −71.28 and 51.3% higher on average; and the heat transfer factor was approximately 135.3 and 105.9% higher on average. In addition, a simulation model was established. The positive correlation between the simulated results and the experimental data demonstrated the effectiveness of the model. With this model, the heat transfer process of ANCRF was simulated, with results indicating that the structure sizes of fin thickness, fin height, fin spacing, and fin angle all have great influence on the heat transfer and flow performance of ANCRF, and the rules of influence are different. This research has a guiding significance for the optimization design and application extending for this new type of fin tube.

Entransy analysis on the thermal performance of flat plate solar air collectors

Based on the thermo-electric analogy (the so-called thermal entransy analysis), the unified airside convective heat transfer coefficient for different sorts of flat plate solar air collectors (FPSACs) is identified in terms of collector aperture area. In addition, the collector thermodynamic characteristic matching coefficient is defined to depict the matching property of collector thermal performance between the collector airside heat transfer and the total heat losses. It is found that the airside convective heat transfer coefficient can be experimentally determined by collector thermal performance test method to compare the airside thermal performances of FPSACs with different types of airflow structures. Moreover, the smaller the collector thermodynamic characteristic matching coefficient is, the better the thermodynamic perfect degree of a FPSAC is. The minimum limit value of the collector thermodynamic matching coefficient is close to zero but it can not vanish in practical engineering. Parameter sensitivity analysis on the total entransy dissipation and the entransy increment of a general FPSAC is also undertaken. The results indicate that the effective way of decreasing total entransy dissipation and enhancing the useful entransy increment is improving the efficiency intercept of the FPSAC. This is equivalent to the cognition result of thermal analysis. However, the evaluation indices identified by the thermal entransy analysis can not be extracted by singular thermal analysis.

Experimental investigations on thermal performance of phase change material – Trombe wall system enhanced by delta winglet vortex generators

Experiments have been performed on the thermal behavior of a PCM (phase change material) – Trombe wall system with heat transfer enhancement by DWVGs (delta winglet vortex generators). PCM (CaCl2·6H2O) slabs were attached on the gap – side wall surface to increase the heat storage. Delta winglet VG pairs (attack angle β = 45°, ratio of length to height l/h = 2) were arranged on the surface of PCM panel to enhance the air side convective heat transfer in the gap. The VG height, pitch and locations were also evaluated. The results showed that the delta winglet VG pairs really enhanced the heat transfer rate at the surface of PCM panel during both the charging and discharging processes. Accordingly, the gap air flow rate and heating rate to the room with a single row of delta winglet VG pairs are 28.5% and 39.4% higher than the case without VGs. Delta winglet VG pairs with height of 30 mm, front edge pitch of 2 mm, lateral spacing of 225 mm and vertical spacing of 340 mm give best thermal performance under present conditions. The results indicate the advantages of using longitudinal vortex generators to improve the performance of PCM – Trombe wall systems for passive solar heating.

Numerical model of a small-scale liquid-to-air membrane energy exchanger: Parametric study of membrane resistance and air side convective heat transfer coefficient

A small-scale single-panel liquid-to-air membrane energy exchanger (LAMEE) is a novel membrane based energy exchanger that allows heat and moisture transfer between a supply air stream and a salt solution flow through a semi-permeable membrane. In this paper, the steady-state effectiveness of the small-scale single-panel LAMEE is experimentally measured and numerically modeled. The numerical model is compared with the experimental data for summer and winter test conditions. Parameters those are investigated using the numerical model include membrane vapor diffusion resistance (VDR) and air side convective heat transfer coefficient. The membrane VDR in the small-scale LAMEE is measured and the effect of this parameter is studied for the steady-state effectiveness. Decreasing the membrane VDR from 56 s/m to 24 s/m increases the LAMEE latent effectiveness by 11%, and as expected, it doesn't have any effect on the LAMEE sensible effectiveness. Enhanced convective heat transfer coefficient in the air flow of the small-scale LAMEE is calculated, and the effect of this parameter on the LAMEE effectiveness is investigated. The results show 138% improvement in the convective heat transfer coefficient in the air channel of the LAMEE with an air screen at Reair = 1570 which results in 11% improvement in the LAMEE effectiveness. Finally, the modified numerical model is validated by the experimental data for the steady-state effectiveness of the small-scale LAMEE under summer test conditions at four different values of the thermal capacity ratio (Cr* = 1, 3, 5 and 7) and NTU = 3.8. Moreover, the numerical model is used for winter test conditions, and the results are compared with the experimental data. The results show that the numerical model agrees with the experimental data for the latent and total effectiveness of the LAMEE under the air heating and humidifying and air heating and dehumidifying test conditions.

Experimental study on the performance of a novel fin-tube air heat exchanger with punched longitudinal vortex generator

To enhance the heat transfer of fin-tube surface, two kinds of novel fin-tube surfaces with two rows of tubes in different diameter, arranged in staggered pattern are presented. Along the direction of air flow, the diameter of the first transverse row of tubes is smaller than that of the second. Delta winglet pairs working as longitudinal vortex generators are punched only around the first transverse row of tubes in reduced size, with either “common flow up” or “common flow down” orientation. The performances of two new types of air heat exchangers using the novel fin-tube surfaces (referred as SA and SB) and one traditional plain fin-tube heat exchanger (named BL) acting as baseline in contract are experimentally studied under dry-surface conditions. On the condition of the minimum cross velocity in heat exchanger umax=4.0m/s, air-side convective heat transfer coefficients of SA and SB increases by 16.5% and 28.2%, respectively, in comparison with that of BL. The air-side pressure drop decreases less than 10% for SA, and slightly decreases for SB, compared with the baseline heat exchanger. The present results show that one can achieve heat transfer enhancement accompanying with a reduction of pressure loss using the strategies of fin-tube surface, longitudinal vortex generator design and appropriate placement on the fin-tube surfaces.

Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger

The present study investigated the effect of perforated circular finned-tube (PCFT) on the convective heat transfer performance of circular finned-tube heat exchangers. The air-side convective heat transfer coefficients increased by 3.55% and 3.31% for 2-hole and 4-hole PCFT cases, respectively. The increase in the convective heat transfer coefficient was related to the reduction of the recirculation region by introducing the perforations at the flow-separation locations on the finned tube. The pressure drop across the finned-tube bundles increased by 0.68% and 2.08% for the 2-hole PCFT and 4-hole PCFT cases, respectively. The greater pressure drop in the case of the 4-hole PCFT might be due to excessive flow disturbances produced by multiple perforations. The fin factor defined as the ratio of the % increase of the convective heat transfer coefficient and that of the pressure drop was 5.19 for the 2-hole PCFT case, whereas that was 1.59 for the 4-hole PCFT case.

원형관 및 납작관-평판휜 형상의 밀집형 열교환기에 대한 대류열전달 상관관계식

본 연구에서는 열의 수가 세 개인 원형관 및 종횡비가 고려된 납작관-평판휜 형상의 밀집형 열교환기 내 공기의 유동 및 열전달 특성을 RNG k-ε 난류모델을 적용하여 수치적으로 해석하였다. 수치조건의 범위는 원형관인 경우 1,000＜Re<SUB>Dh</SUB>＜10,000이며, 납작관인 경우 1,000＜Re<SUB>Dh</SUB>＜10,000 및 3.06＜AR<5.44이다. 마찰인자는 ?=CRe<SUP>m</SUP><SUB>Dh</SUB>의 형태로, Colburn j 계수는 j=Cre<SUP>m</SUP><SUB>Dh</SUB> 형태로 상관관계식을 제시하였다. 이러한 상관관계식은 향후 원형관 및 납작관-평판휜 형상의 열교환기 설계시에 기본적인 자료로 충분히 활용될 수 있을 것으로 판단된다.

비대칭 엇갈림 배열로 구성된 납작관-평판휜 형상의 밀집형 열교환기에 대한 대류열전달 특성 해석

Abstract Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in compact heat exchanger with flat tubes and continuous plate fins having a symmetric and non-symm etric staggered arrangements. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous experimental results. In order to investigate the flow and heat transfer features by periodic boundary conditions, the three blocks were used. Predicted heat transfer coefficients between the three blocks are similar while there are relatively differences, compared with the experimental data. From the calculated results a correlation for Colburn j factor in the compact heat exchanger system is suggested. The predicted results in this study can be applied to the optimal design of air conditioning system. 1. 서 론 단위 체적 당 매우 큰 열전달 표면적이 요구되는 밀집형 열교환기(compact heat exchanger)는 주로 열전달매체가 기체일 경우에 사용되며 표면의 휜(fin)과 판(plate)의 형상에 따라 여러 종류로 분류된다. 대표적인 형태로써 납작관-연속적인 평판휜(flat tubes-continuous plate fins), 원형관-연속적인 평판휜(circular tubes-continuous plate fins), 원형관-원형휜(circular tubes-circular fins) 등이 있

Experimental measurement and numerical computation of the air side convective heat transfer coefficients in a plate fin-tube heat exchanger

The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k - ɛ turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082∼1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%.

종횡비에 따른 납작관-평판휜 형상의 밀집형 열교환기 내공기 측 대류열전달특성에 대한 수치해석

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in a compact heat exchanger with flat tubes and continuous plate fins according to the aspect ratio. RNG k-ε model is applied for turbulence analysis. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous correlations for circular tubes. The numerical conditions are considered for the aspect ratios ranging from 3.06 to 5.44 and Reynolds number ranging from 1000 to 10,000. The results showed that heat transfer coefficients decreased with the increase of aspect ratio. From the calculated results a correlation of Colburn j factor for the considered aspect ratio in the compact heat exchanger system is suggested. The predicted results in this study can be applied to the optimal design of air conditioning system.

원형관-평판휜 형상의 밀집형 열교환기 내 공기 측대류열전달특성에 대한 수치해석

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in a compact heat exchanger with circular tubes and continuous plate fins. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous experimental correlations. Three models of standard and RNG , and Reynolds stress are applied for turbulence model applicability. Predicted heat transfer coefficient from the models of standard and RNG are very close to those of the heat transfer correlations while there are relatively large difference, more than 17 percentage in the result from the Reynolds stress model. From the calculated results a correlation for Colburn j factor in the compact heat exchanger system is suggested.

Air-side data interpretation and performance analysis for heat exchangers with simultaneous heat and mass transfer: Wet and frosted surfaces

Data reduction methods for wet- and frosted-surface heat exchangers are reviewed, and logarithmic-mean temperature difference (LMTD) and logarithmic-mean enthalpy difference (LMED) methods for data interpretation are formulated, including fin efficiency expressions for flat-tube heat exchangers with constant-area fins. With these improved formulations for data interpretation in place, the two methods are compared using a numerical simulation of a flat-plate heat exchanger. For all conditions considered—spanning wet and frosted surfaces—the new UA-LMTD method provides the air-side convective heat transfer coefficient to within 3% and is more accurate than the HA-LMED method, which results in large errors under some conditions.

An Experimental Investigation of Convective Heat Transfer From Wire-On-Tube Heat Exchangers

An experimental investigation of the air-side convective heat transfer from wire-on-tube heat exchangers is described. The study is motivated by the desire to predict the performance, in a forced flow, of the steel wire-on-tube condensers used in most refrigerators. Previous investigations of wire-on-tube heat exchangers in a forced flow have not been reported in the literature. The many geometrical parameters (wire diameter, tube diameter, wire pitch, tube pitch, etc.), the complex conductive paths in the heat exchanger, and the importance of buoyant forces in a portion of the velocity regime of interest make the study a formidable one. A key to the successful correlation of the experimental results is a definition of the convective heat transfer coefficient, hw, that accounts for the temperature gradients in the wires as well as the vast difference in the two key characteristic lengths—the tube and wire diameters. Although this definition results in the need to solve a transcendental equation in order to obtain hw from the experimental data, the use of the resulting empirical correlation is straightforward. The complex influence of the mixed convection regime on the heat transfer from wire-on-tube heat exchangers is shown, as well as the effects of air velocity and the angle of attack. The study covers a velocity range of 0 to 2 m/s (the Reynolds number based on wire diameter extends to 200) and angles of attack varying from 0 deg (horizontal coils) to ±90 deg. Heat transfer data from seven different wire-on-tube heat exchangers are correlated so that 95 percent of the data below a Richardson number of 0.004, based on the wire diameter, lie within ±16.7 percent of the proposed correlation.