Non-uniform Air Velocity Research Articles

Experimental and numerical research on a C-type heat exchanger in duct air conditioner under non-uniform airflow

The indoor unit of duct air conditioner typically employs an A-type heat exchanger (HX), but non-uniform airflow within the duct significantly degrades heat transfer performance. The current study proposes a C-type HX to accommodate the non-uniform airflow better. Experiments are conducted to compare the C-type and A-type HXs under varying conditions, including air volume flow rate, air velocity non-uniformity, inlet air temperature, and condensing temperature. Results indicate that as the non-uniformity of inlet air velocity increases, the heat transfer capacity of A-type HX decreases obviously. In contrast, the C-type is insensitive to the non-uniform air velocity distribution, which consistently outperforms the A-type in terms of heat transfer capacity throughout various operating conditions. Additionally, a numerical model is developed to compare the local parameters of the two HX types. The C-type demonstrates a more uniform outlet air temperature and exhibits a heat transfer capacity that is 19.1% higher than the A-type, while maintaining the identical heat transfer area and size parameters.

Design and experimental validation of a new condenser of an automotive air conditioning unit to address non-uniform air velocity distributions

• The front anti-collision beam in a car leads to air maldistribution. • A six-path condenser is an optimization of the traditional four-path condenser. • The new condenser shows better capacity with the anti-collision beam. • The cooling performance of automobile air conditioning system is improved. To improve the efficiency of automobile air conditioning and reduce the impact of non-uniform air velocity distributions caused by the front anti-collision beam on automobiles, we designed a new six-process condenser and tested it under different working conditions. A baffle was positioned to simulate the anti-collision beam during the experiment. We analyzed the condenser by itself and the system level performance of this newly designed condenser under three working conditions compared to a traditional four-process condenser. Condenser heat exchange capacity, condenser inlet pressure, system refrigerant flow rate, refrigerant flow resistance, thermal imaging of condenser outlet air, overall system cooling capacity, and coefficient of performance were compared. The coefficient of performance of the system with this new condenser improved by 8.5%, 7.8%, and 10.3% for the three tested working conditions.

The Determination of the Thermal Characteristics of the Microchannel Heat Exchangers for Single-Phase R600a Flow

Bu çalışmada, özellikle iklimlendirme sistemlerinde ön ısıtma/soğutma, aşırı kızdırma/soğutma gibi tek fazlı soğutkan akışının olduğu uygulamalar için mikrokanal eşanjörlerin matematiksel modellemesi deneysel doğrulamalı olarak ele alınmıştır. Çalışma akışkanının R600a olduğu panjurlu kanatlı mikrokanal eşanjörler için R600a’nın çıkış sıcaklığını, toplam ısı transfer kapasitesini ve entropi üretimini tahmin eden bir matematiksel benzetim modeli geliştirilmiştir. Modelin doğruluk hassasiyetini arttırmak için geçişlerdeki farklı kütle hızları ve uniform olmayan hava hızı modelde dikkate alınmıştır. Bu etkileri dikkate almak için literatürde yer alan diğer modellerden farklı olarak iki seviye ayrıklaştırma ile oluşturulan bir hesaplama sistemi modelde uygulanmıştır. Modelde mikrokanal ısı eşanjörünün ön yüzü hava akış bölgelerine bölünerek uniform olmayan hava hızları dikkate alınmıştır. Model sonuçlarını doğrulamak için deneysel çalışma yapılmıştır. Deneysel doğrulama sonucunda modelin, incelenen tüm test koşulları için çıkış sıcaklığını ±%10 aralığında bir ortalama mutlak sapma ile öngördüğü sonucuna varılmıştır. Mikrokanal ısı eşanjörünün, ısı transfer performansını tahmin etme kabiliyeti, deneylerle değerlendirilmiş, uniform olmayan hava hızının modele dahil etmenin, modelin doğruluk hassasiyetini arttırdığı görülmüştür. Mikrokanal ısı eşanjöründeki entropi üretim mekanizmaları incelenmiş ve akışkan akımı tersinmezliklerinin entropi oluşumuna katkısının, ısı transfer tersinmezliklerine kıyasla oldukça düşük olduğu tespit edilmiştir.

Performance prediction of hydronic cooling coil with non-uniform air velocity (ASHRAE RP-1741)

Hydronic coils are often exposed to non-uniform air velocities due to a series of factors, including tight spacing between a fan and a coil, flow area contraction and expansion, existence of obstructions, abrupt change in airflow direction, or the combination of above factors. Although studies showed non-uniform air velocities could cause performance degradation for direct expansion (DX) cooling coils, efforts in quantifying this impact on hydronic cooling coils are limited. The objective of this paper is to develop and validate a section-by-section calculation procedure for cooling performance prediction of hydronic coils with non-uniform air velocities. As the first step, air velocities at the coil face in a hydronic fan coil unit (FCU) were measured using a vane-type anemometer. The coil was then discretized into multiple sections, with each section using the measured air velocity for heat and mass transfer analysis. The cooling performance prediction with non-uniform air velocities was validated against experimental results. The comparative analysis showed that capturing the impact of non-uniform air velocities decreased the normalized root mean square error (RMSE) from 39.9% to 8.1% for latent capacity prediction and 9.1% to 2.6% for total capacity prediction compared with the results assuming uniform air velocities.

Numerical and experimental study on thermal characteristics of louvered fin microchannel air preheaters

Microchannel heat exchangers have been gradually getting importance in industrial applications due to offering outstanding benefits. The current study has focused on the development of a numerical model to predict the thermal performance of the microchannel air preheaters (MCPH) for HVAC systems. An experimental study has been performed to validate the numerical model results. A louvered fin multiport microchannel heat exchanger has been employed as an air preheater in the experiments. The proposed model has been developed based on the segment-by-segment approach and calculated the outlet temperature and heat capacity of the MCPH. Different air velocities at the frontal face and varying mass flow rates in passes of the MCPH have been taken into consideration in the model. It has been concluded from experimental data that the model predicts the outlet temperature with an average absolute deviation within ±2% for all investigated test conditions. The proposed model shows high accuracy with respect to temperature calculation. Another conclusion is that the non-uniform air velocity approach improves the precision of the proposed model. The heat capacity predictions with the uniform air velocity approach indicate higher deviations than the non-uniform air velocity approach.

Effects of Airflow Profile and Condensation Pressure on Performance of Air-Cooled Condensers

The effect of airflow profile and condensation pressure on the performance of air-cooled condensers is investigated experimentally. Two large, flattened-tube air-cooled steam condensers are studied. The tube lengths are 10.7 and 5.7 m, with inner dimensions of 216 × 16 mm and aluminum fins on each side of the elongated-slot cross sections. Capacity and pressure drop are measured and discussed here. All tests are performed with a horizontal tube and co-current vapor and condensate flow. Four different profiles of cross-flowing air are tested: uniform air flowing upwards, uniform air flowing downwards, and two profiles of non-uniform air flowing upwards. For the 10.7 m tube, reversing airflow direction from upwards to downwards is found to significantly increase condenser capacity. Also for the 10.7 m tube, a favorable non-uniform air-velocity profile is shown to increase capacity in comparison to a uniform air-velocity profile. Both of these performance increases are shown to be the result of matching regions of maximum heat transfer coefficient on the air and steam sides. For the 5.7 m tube, a non-uniform airflow profile is shown to have no effect on capacity. Reducing condensation pressure is shown to decrease condenser capacity for both condenser tubes.

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

Estimating the non-uniform air velocity distribution for the optimal design of a heat exchanger

The maldistribution of flow in a heat exchanger usually has negative effects on the heat exchanger specifications such as the effectiveness, annual cost, and pressure drop. This paper examines the issue to improve the thermoeconomic performance. For this purpose, a plate-fin heat exchanger was selected and optimized by considering both the effectiveness and total annual cost as objective functions. The design parameters were the parameters of the heat exchanger and fin, as well as the non-uniform inlet flow profiles for both sides of the hot and cold streams. To generalize the results, the optimization was applied at different mass flow rates. The optimum results of the non-uniform profile were compared with those obtained with a uniform profile of the inlet flow. The optimum results reveal that thermoeconomic improvement occurred in the case of non-uniform flow compared with the case of uniform flow. For example, a 12.7% reduction for the total annual cost was found in the case of non-uniform flow in comparison to the uniform flow for a fixed effectiveness of 0.92. At low effectiveness, no significant improvement was found in the case of non-uniform flow. Nevertheless, the suggested profiles could be considered as an alternative to uniform inlet profiles.

A new approach to mitigate intense temperature gradients in ceramic foam solar receivers

Abstract The present study aims to present an approach to mitigate the maximum solid temperature and its gradient inside the porous material in volumetric solar receivers. To this end, a porous receiver with non-uniform air velocity at the inlet is considered in the present study. Comparison of the results with those obtained for a porous receiver with uniform air velocity at the inlet reveals the ability of the new velocity distribution in reducing the maximum solid temperature and its gradient within the solid phase. The temperature distribution is obtained for different porosities and pore diameters in the porous receivers with uniform and non-uniform air velocity distributions at the inlet. It is observed that the proposed distribution of air velocity at the inlet decreases the maximum solid temperature within the porous receiver even for small porosities and pore diameters of porous media.

Heat transfer performance variations of condensers due to non-uniform air velocity distributions

The heat transfer performance variations of condensers that are caused by non-uniform distributions of air flows are investigated using a numerical simulation method. A heat exchanger designed for the outdoor unit of a heat pump system is selected and represented using a numerical model. A non-uniform profile of the air velocity is constructed through measuring the air velocity at various locations on the outdoor unit. Numerical analyses are conducted for various refrigerant circuits and air flow conditions. The results demonstrate that the heat transfer capacity is reduced depending on the air flow rate and the refrigerant circuit configuration. It is also demonstrated that the capacity reduction rate is increased as the average air velocity decreases.

Aerodynamic drag of radiator in unsteady air flow

Nonuniform distribution of velocity of cooling air over the front surface of a radiator increases its drag. The paper analyzes the factors affecting the increase of aerodynamic drag under the influence of unsteady air flow. The degree of drag change depends on aerodynamic properties of radiator core, namely, the curvature of the graph of aerodynamic characteristics of a radiator and nonuniform air velocity field over the front of a radiator. In actual use, increase of drag of a radiator can be more than 20%.

Effect of non-uniform air velocity distribution on evaporator performance and its improvement on a residential air conditioner

The effect of non-uniform air velocity distribution on performance of a multi-circuit evaporator was studied numerically and experimentally. The non-uniform air velocity distribution on evaporator was measured experimentally, and its corresponding evaporator performance was tested and approximated numerically. The evaporator performance was also investigated numerically at the equal airflow rate but under uniform air distribution. The results showed that the evaporator capacity under non-uniform air distribution was decreased by 7.78% than that under uniform air distribution. The decrease in evaporator capacity was mainly attributed to smaller overall heat transfer coefficient of lower tubes with smaller air velocity in first row because overall tube heat transfer coefficient varied remarkably at smaller air velocity and gently at greater air velocity. Two triangle air guide plates were installed on sheet backing of evaporator to deflect part of airflow towards evaporator bottom to increase air velocity of lower tubes and to decrease the area that smaller air velocity covered. The experimental results showed air guide plates decreased blend loss of high temperature air leaving upper circuits and low temperature air leaving lower circuits, thus decreasing outlet air dry and wet temperatures. Finally, the cooling capacity and EER were increased by 3.02% and 5.1%, respectively. ► Evaporator performance between non-uniform and uniform air distributions are compared. ► Evaporator capacity under non-uniform air distribution decreased by 7.78%. ► Overall heat transfer coefficient was a dominant factor of evaporator capacity decrease. ► Air guide plates on sheet backing of evaporator could improve evaporator performance.

Optimization of guide vane positions in bended inflow of mechanical draft wet-cooling tower

Optimization of guide vane positions in bended inflow of mechanical draft wet-cooling tower Optimization of vane positions in a mechanical draft wet-cooling tower is presented in this paper. The originally installed, equally spaced, vanes produced non-uniform air velocity distribution reducing the performance of the fill of the cooling tower. A 2D CFD model of the tower has been created. The model has then been used to determine the objective function in the optimization procedure. The selected objective function was the standard deviation of the velocity of air entering the fill. The Goal Driven Optimization tools of the ANSYSWorkbench 2.0 have been used for the optimization and the ANSYS Fluent 13.0 as a flow solver. The optimization allowed reduction of the objective function and producing a more uniform air flow.

An analytical study on heat transfer performance of radiators with non-uniform airflow distribution

Heat exchangers used in modern automobiles usually have a highly non-uniform air velocity distribution because of the complexity of the engine compartment and underhood flow fields; hence ineffective use of the core area has been noted. To adequately predict the heat transfer performance in typical car radiators, a generalized analytical model accounting for airflow maldistribution was developed using a finite element approach and applying appropriate heat transfer equations including the ε-NTU (effectiveness - number of heat transfer units) method with the Davenport correlation for the air-side heat transfer coefficient. The analytical results were verified against a set of experimental data from nine radiators tested in a wind tunnel and were found to be within +24 and −10 per cent of the experimental results. By applying the analytical model, several severe non-uniform velocity distributions were also studied. It was found that the loss of radiator performance caused by airflow maldistribution, compared with uniform airflow of the same total flowrate, was relatively minor except under extreme circumstances where the non-uniformity factor was larger than 0.5. The relatively simple set of equations presented in this paper can be used independently in spreadsheets or in conjunction with computational fluid dynamics (CFD) analysis, enabling a full numerical prediction of aerodynamic as well as thermodynamic performance of radiators to be conducted prior to a prototype being built.

Thermal Performance Reduction in Air-Cooled Heat Exchangers Due to Nonuniform Flow and Temperature Distributions

The effect of nonuniform (or maldistributed) inlet airflow and temperature on the thermal performance of cross-flow air-cooled heat exchangers, with both fluids unmixed, and taking into consideration differences in performance characteristics of individual tube rows, is addressed in this article. The latter may be due to different fin pitches or other geometric variations or row effects due to changes in air turbulence. Downstream tube rows in such a multirow tube bundle experience higher performance reductions than the tube row at the air inlet. A nonuniform air velocity distribution to a tube row leaves the row with a distorted temperature profile. This temperature nonuniformity increases as the air passes through subsequent tube rows, and causes the downstream rows to be progressively less effective. The present analysis is used to evaluate the performance of the individual tube rows and ultimately the entire bundle. Velocity distributions which have been measured on air-cooled heat exchanger models ar...

Effect of laden solid particles on the turbulent flow structure of a round free jet

The effect of solid particles on the flow structure of a round air jet in a stagnant surrounding was investigated experimentally. Information on the averaged two-component velocities, the kinetic energies, and the u′ v′-properties were obtained for both phases by means of a monochromatic three beam laser Doppler anemometer. The particle number density was also measured by this system. Glass beads of 64 μm and 132 μm diameter were used for a constant mass loading ratio of 0.3 in a jet with a Reynolds number of 20 000. The lateral mean velocity and number density profiles were expressed by best fitting functions and several invariable coefficients were found. The standard drag force coefficient C D for a single particle was applicable for a dilute particle cloud even in a non-uniform air velocity field.