Multiport Tube Research Articles

Flow boiling heat transfer coefficient and pressure drop of R448A inside various multiport tubes: An experimental investigation and correlation assessment

R448A, a safety class A1 a-zeotropic refrigerant blend with a GWP of 1390, is considered an efficient and environmentally friendly substitute for R404A in direct-expansion commercial refrigeration systems. Herein, we experimentally investigate the flow boiling heat transfer and pressure drop of R448A inside a multiport mini-channel tube. The experimental ranges of mass flux and heat flux are 100–500 kg/m2s and 3–15 kW/m2, respectively, and they cover the entire quality range at three different saturation temperatures of 3, 6, and 10°C. Three different test tubes are utilized in this study to evaluate the effect of tube geometry on the heat transfer, and their diameters range from 0.831 to 1.14 mm. The results show that mass flux has favorable effects on the heat transfer coefficient and pressure drop, while heat flux has a favorable effect on the heat transfer coefficient at low mass fluxes. As the saturation temperature decreases when the mass flux is high, the heat transfer coefficient and pressure drop increase. The tube with largest perimeter but the smallest hydraulic diameter has the highest heat transfer coefficient under the low mass flux condition. Heat transfer coefficient and pressure drop experimental data were compared with the models in literature, and the results of our statistical assessment indicate that the pure fluid model modified to account for the mixture effect produces the best results. Meanwhile, most pressure drop models produce reasonably good predictions. Additionally, a new correlation is proposed to predict the heat transfer coefficient in practical applications.

Boiling heat transfer of binary and ternary mixtures in multiple rectangular microchannels

In this study, the boiling heat transfer of binary and ternary refrigerant mixtures in a horizontal multiport tube with multiple rectangular microchannels was investigated. The influences of the mass velocity, vapor quality, and heat flux on the heat transfer were evaluated for near- and non-azeotropic mixtures of R410A, R32/CF3I, R32/R1123, R32/R1234yf, R1123/R32/R1234yf, and CO2/R32/R1234yf. The obtained results were compared with those of pure refrigerants R1234yf and R32. For R1234yf at low mass velocity, the flow patterns were estimated as intermittent (plug) and slug-annular flows with an extremely thin liquid film. The heat transfer coefficient decreased when the heat flux was increased because dry patch areas formed and expanded on the sides of the noncircular microchannel. The near-azeotropic mixtures with small temperature glides exhibited heat transfer characteristics similar to those of the pure refrigerant. Conversely, the heat transfer coefficients deteriorated slightly under lower mass velocities. The heat transfer coefficients of binary and ternary mixtures with larger temperature glides were significantly different from those of pure refrigerants and near-azeotropic mixtures; they decreased with decreasing mass velocity owing to mass diffusion resistance. The heat transfer coefficients of binary mixtures with large temperature glides decreased monotonically as the mass velocity decreased, and the heat transfer noticeably deteriorated at lower mass velocities. The heat transfer model was developed for rectangular microchannels by considering the heat transfer contributions of thin liquid film evaporation due to surface force, forced convection, and nucleate boiling. Moreover, this model considers the flow pattern of multiple microchannels, heat transfer degradations due to dry patches and mass diffusion resistance, and dryout incipience quality. The results of the present model agree well with the heat transfer coefficients, including those of binary and ternary mixtures, in the pre- and post-dryout regions with a mean percentage error of 3.1%.

Condensation heat transfer of binary and ternary mixtures inside multiport tubes

This study investigated the condensation heat transfer of pure refrigerants and binary and ternary mixtures inside horizontal multiport tubes with multiple square minichannels. The effects of mass flux, vapor quality, heat flux, channel size, and channel shape were evaluated using pure refrigerants R32 and R1234yf; binary mixtures R32/R1234yf, R32/R1123, R32/R13I1, and R410A; and ternary mixtures R1123/R32/R1234yf and CO2/R32/R1234yf. For pure refrigerants, the heat transfer coefficient in the annular and churn flow regimes decreased monotonically with decreasing vapor quality. By contrast, the heat transfer coefficient in the plug flow regime was higher over a wide vapor quality range because of the thin condensate film formed by surface tension. Smaller channels exhibited higher heat transfer coefficients. The square minichannels exhibited higher heat transfer coefficients than the circular minichannels, except in the region with high mass flux and high vapor quality. For binary and ternary mixtures, the heat transfer coefficients significantly decreased with decreasing mass flux and vapor quality and were strongly deteriorated at lower mass fluxes. Compared with the annular and churn flow regimes, in the plug and slug-annular flow regimes, where heat transfer through the thin liquid film was dominant, the effect of mass diffusion resistance in deteriorating heat transfer was the most pronounced. A new heat transfer model for pure refrigerants inside multiport tubes with multiple rectangular minichannels was developed considering the contributions of vapor shear stress and surface tension. The proposed model was verified using a dataset of pure refrigerants, and a mean absolute percentage error (MAPE) of 11.5% was achieved. Furthermore, a new heat transfer model that considers the heat transfer deterioration of non-azeotropic refrigerant mixtures was proposed, which could predict 508 datapoints with an MAPE of 13.3%.

Condensation heat transfer and pressure drop characteristics of low GWP R-404A alternative refrigerants in a multiport tube having axial microfins

Considering the phase out of the hydrofluorocarbons (HFCs), the search for an alternative refrigerants is of an urgent issue. In this study, four R-404A alternative refrigerants (R-448A, R-449A, R-455A, R-454C) were tested in a microfin multiport tube having 1.30 mm hydraulic diameter. The smooth mutiport tube of 0.8 mm hydraulic diamter was also tested for a comparison purpose. The results showed that, at a low mass flux and low vapour quality, the microfin tube yielded approximately equal heat transfer coefficients to those of the smooth tube. At a high mass flux and high vapour quality, however, the heat transfer coefficient trends were different depending on the refrigerants. For R-404A, which has a negligible temperature glide of 0.31 K (at 45 °C saturation temperature), the heat transfer coefficients of the two tubes were approximately the same. However, for R-454C and R-445A, which have the large temperature glides of 6.59 K and 9.46 K, the heat transfer coefficients of the microfin tube were smaller than those of the smooth tube. Based on the trends of the heat transfer coefficients of this study, three criteria for a possible good correlation are proposed. First, the correlation should predict the heat transfer coefficients of pure (or near azeotropic) refrigerants in a smooth tube adequately. Second, the correlation should predict the heat transfer coefficients of pure (or near-azeotropic) refrigerants in a microfin tube smaller than those in a smooth tube. Third, the underprediction of the microfin data should get weakened as the temperature glide increases. Based on the above criteria, it was found that some micro-channel or conventional channel correlations may be used to predict the condensation heat transfer coefficients in a multiport tube with/without microfins. The pressure drops were generally in the reverse order to the value of vapour density. Furthermore, the pressure drops of the alternative refrigerants were larger than those of R-404A.

Evaporation heat transfer and pressure drop of low GWP R-404A alternative refrigerants in a multiport tube

Considering the phase out of the hydrofluorocarbons (HFCs), the search for an alternative refrigerants is of an urgent issue. In this study, four R-404A alternative refrigerants (R-448A, R-449A, R-455A, R-454C) were tested in a multiport tube having 1.61 mm hydraulic diameter. The tests were conducted changing the mass flux from 200 to 400 kg/m2s, heat flux from 2.8 to 6.5 kW/m2 at the saturation temperature of 15 °C. The results showed that the heat transfer coefficients of R-404A were higher than those of the alternative refrigerants. However, the pressure drops of R-404A were smaller than those of those of the alternative ones. Possible explanations are provided based on the thermophysical properties and the temperature glides amongst refrigerants. At a low heat flux, the heat transfer coefficient increased as the mass flux or vapour quality increased. At a high heat flux, however, the heat transfer coefficient was relatively independent of the mass flux or vapour quality. This suggested that both the nucleate boiling and the convective heat transfer contributed to the heat transfer in the present multiport tube. Comparison with the existing correlations revealed that the mixture correlation of Shah [53] reasonably predicted the heat transfer coefficients with the RMSE of 0.16, although the trends between the data and the predictions were somewhat different. The pressure drop data were reasonbly predicted by several mini-channel correlations with the RMSE of 0.16.

Air-Cooled Loop Thermosyphon Cooling System for High Heat Load CPUs—Part I: Design and Performance Simulation

This article proposes and evaluates the thermal-hydraulic performance of a compact, lightweight, and high heat-transfer capacity cooling system for central processing unit (CPU) temperature control. The cooling loop, defined as a closed loop thermosyphon (LTS), has a passive flow driven mainly by the buoyancy in the riser. It is a self-modulated system, i.e., without heat flow ceases but it increases with increasing heat load. The LTS is a partially passive system since energy is still required to drive the fan blowing air through its condenser. The main advantage relative to conventional forced air-cooled heat spreaders is its ability to transfer much higher levels of heat while consuming less fan power. The innovative patented LTS system has two thermosyphon loops working in parallel, composed of a multimicrochannel evaporator, a multiport tube louvered fin condenser, two risers, and one downcomer. The simulations show that it is able to cool 600 W from a 50 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 50 mm CPU footprint without dryout or surpassing the critical heat flux. It was specifically simulated for a CPU of 2U server, considering a height of only 66 mm and width of 214 mm. Thus, two of these units can be installed side-by-side in a 2U server, which means a total cooling capacity of 1200 W per server. Several simulations are presented showing the high thermal-hydraulic performance of the LTS and a case study showing the reduction of fan energy consumption when comparing the proposed LTS with an actual air-cooled heat spreader system. The LTS was prototyped, experimentally tested, and its simulations successfully validated in Part II of this study.

Condensation heat transfer and pressure drop of low GWP R-404A alternative refrigerants in a multiport tube

The literature survey reveals that condensation or evaporation studies on R-404A alternative refrigerants are very limited. Especially for the multiport tube, no prior investigation exists. In this study, four R-404A alternative refrigerants (GWP100≤2500: R-448A, R-449A and GWP100≤150: R-455A, R-454C) - including R-404A (GWP100 = 3922) were tested in a multiport tube having 0.80 mm hydraulic diameter. The tests were conducted changing the mass flux from 380 to 760 kg/m2s, vapor quality from 0.2 to 0.8. During the tests, the saturation temperature and the heat flux were maintained at 45 °C and at 6 kW/m2. The results showed that the heat transfer coefficients of the alternative refrigerants were higher than those of R-404A. The reason was attributed to the better thermo-physical properties of the alternative refrigerants. Especially, at a high mass flux of this study, the gas core and the liquid film became turbulent, which mitigated the malign effect of temperature glide by strong mixing. Hence, the alternative refrigerants yielded higher heat transfer coefficients than R-404A. Some correlations with mixture correction predicted the heat transfer coefficient reasonably well with RMSE of approximately 0.20. An interesting finding is that even better predictions (RMSE of 0.13) were obtained when the correlation was not mixture corrected. The pressure drops were generally in the reverse order to the value of vapor density. Pressure drops of alternative refrigerants were larger than those of R-404A. The pressure drop correlations generally underpredicted the data.

Flow condensation of R-410A in multiport tubes having smooth or micro-finned internal channels

ABSTRACT Three micro-finned and two smooth multiport tubes having the same tube width and height with the microfin tubes were tested using R-410A. For the smooth multiport tube, the tube with smaller hydraulic diameter yielded both higher heat transfer coefficients and larger pressure drops. However, for the microfin tube, results suggested that, fin configuration (number of fins per channel, fin height, etc.) has to be considered in addition to hydraulic diameter. The trend of the pressure drop generally followed the hydraulic diameter – smaller hydraulic diameter tube yielded larger pressure drop. Data are compared with the predictions by existing correlations.

Flow boiling heat transfer of zeotropic mixture R1234yf/R32 inside a horizontal multiport tube

• Experimental studies of R1234yf/R32 mixtures were conducted. • Flow boiling heat transfer and pressure drop inside multiport tubes were examined. • Heat transfer and pressure drops of mixtures and pure components were compared. • Effects of mass flux, heat flux, and quality on flow boiling were clarified. • A predictive heat transfer correlation for refrigerant mixtures was proposed. The flow boiling heat transfer and pressure drop of zeotropic binary mixture R1234yf/R32 were experimentally investigated inside a horizontal multiport tube with rectangular minichannels. Local heat transfer coefficients were quantified under mass fluxes in the range 50–400 kgm −2 s −1 , heat fluxes in the range 5–20 kWm −2 , and circulation compositions of 80/20 and 50/50mass%. The obtained heat transfer coefficient of the mixtures were compared with those of pure components under the same experimental conditions. The heat transfer of the mixtures was strongly influenced by mass flux, vapor quality, and mass fraction, whereas the influence of heat flux on heat transfer was small. The heat transfer coefficients of the mixtures were lower than those of the pure components under most conditions owing to mass diffusion resistance and temperature glide; however, the heat transfer coefficients of the mixtures were same or higher than those of R1234yf at high mass flux and high vapor quality regions. The frictional pressure drops of the mixtures increased with increasing mass flux, vapor quality, and mass fraction of R1234yf. The database encompassing more than 900 and 190 for boiling heat transfer coefficient and frictional pressure drop were compared with available previous correlations. Previous correlations underestimated the heat transfer coefficients of the mixtures, especially for data with higher temperature glide and dominant forced convective heat transfer. The proposed correlation shows good agreement with the heat transfer coefficients of the R1234yf/R32 mixtures with mean and mean absolute deviations of −5.0% and 10.3%, respectively.

Flow boiling heat transfer and flow distribution of HFC32 and HFC134a in unequally heated parallel mini-channels

Abstract A novel high-performance heat exchanger using multiport extruded tubes which have parallel refrigerant mini-channels has recently been developed for air conditioning systems. In a multiport tube for evaporators, due to unequal heat transfer rates among parallel channels, vapor–liquid two-phase refrigerant flow tends to be distributed unevenly. This flow maldistribution makes it difficult to predict heat exchange performance accurately. Hence, it is important to clarify the characteristics of flow boiling heat transfer and flow maldistribution in unequally heated parallel mini-channels. In the present study, experiments were performed on flow boiling of HFC32 and HFC134a in unequally heated two parallel circular mini-channels connected with T-junction headers. When heat flux was unequal between two channels, mass flow and inlet quality maldistributions were recognized. These maldistributions were more intense with widening inequality in channel heat fluxes. In addition, the effects of the averaged flow rate of the two channels, the quality before branching, and refrigerant properties on flow maldistribution were examined. Based on the obtained characteristics, correlation equations were developed to reproduce the mass flow rates and inlet qualities in the respective channels. In boiling heat transfer, dryout tended to occur in a relatively wide portion of the higher-heated channel because of flow rate reduction. Then, heat transfer became degraded especially near the outlet, even at the same averaged heat flux of two channels as the value at which dryout did not occur under equally heated condition.

Effect of lubricant oil on boiling heat transfer and flow patterns of R32 inside a multiport tube

This study experimentally investigated the boiling flow patterns, local heat transfer coefficients, and pressure drops of R32/lubricant oil mixtures flowing inside a horizontal multiport extruded tube having rectangular minichannels. The experiments were carried out in a mass flux range of 50–400 kg m−2 s−1, heat flux range of 5–40 kW m−2, and oil circulation up to 3.7 wt% at an evaporation temperature of 15 °C. The boiling flow patterns were visualized by using a setup that consisted of horizontal multiple square minichannels, each with a hydraulic diameter of 1.0 mm. The heat transfer coefficients of the R32/oil mixtures were lower compared with those of pure R32 and decreased with increasing oil concentration at lower heat fluxes, while they were higher for conditions of higher heat flux and low vapor quality. In addition, the heat transfer coefficients of the mixtures significantly decreased at a lower mass flux and in a higher vapor quality region. The frictional pressure drops of the R32/oil mixtures was higher compared with that of pure R32 and increased with increasing mass flux, vapor quality, and oil concentration. The effect of oil concentration on the pressure drop was significant at the higher vapor quality region. Furthermore, the pressure drops increased with increasing heat flux for lower to middle vapor quality, in contrast to pure refrigerants.

Condensation heat transfer characteristics and pressure drops of R410A, R22, R32, and R290 in a multiport rectangular channel

Experiments on the two-phase flow condensation heat transfer of R410A, R22, R32, and R290 (propane) inside a multiport minichannel tube are investigated in this study. The experiments are conducted with heat fluxes in the range of 3 to 15 kW/m2, working fluids mass fluxes in the range of 50 to 500 kg/m2s, and vapor quality up to 1.0, at a fixed saturation temperature of 48 °C. The test section is an aluminum multiport minichannel tube with a hydraulic diameter equal to 0.83 mm and 18 rectangular channels. The effects of mass flux, heat flux, and thermal properties of refrigerants on the heat transfer performance and pressure drop are illustrated and analyzed. The experimental heat transfer coefficients are compared with well-known correlations. Finally, from the experimental data, a new condensation heat transfer coefficient correlation for all refrigerants is proposed.

Study on the Pressure Bearing Capability of Folded Multi-Port Flat Tube.

The pressure bearing capability of a folded multi-port flat tube (MPFT), which has the advantage of retaining the corrosion property of corrosion resistant materials, was investigated in this study with both a burst pressure test and finite element simulation. Results show that the folded tube’s failure is mainly caused by the breaking of the inner ribs. Instead of detecting inner pressure, the bulging ratio, which is supposed to be small under service pressure, rises rapidly before failure. Therefore, it is suggested to use bulging ratio to visibly determine the working status of folded MPFTs. Based on FE simulations, the pressure bearing capability of the folded MPFT was improved by optimizing the relevant folding parameters. In addition, the influence of in-plane bending was also investigated. It is found that the folded MPFTs can still retain most of the pressure bearing capability after in-plane bending.

Optimization of die design for thin-walled flat multi-port tube with the aid of finite element simulation

Abstract Die design plays a critical role in aluminum alloy profile production, which directly influences the quality of the final products and the service life of the die. In the present study, a porthole die for a multi-hole extrusion process of a thin-walled flat multi-port tube is designed and optimized through numerical simulations. The simulation results show that with the original die, the extruded tube has a non-uniform velocity distribution in its cross-section and bends outward when exiting the die orifice. Meanwhile, a severe stress concentration in the mandrel teeth causes the premature failure of the die, and an uneven deflection of the mandrel teeth and bearing causes the arc-shaped wall thickness of the tube to exceed the tolerance level, which is validated by corresponding extrusion experiments. To solve these problems, two periods of die structure modifications are proposed, and their extrusion processes are simulated. The optimized die is confirmed to exhibit better performance than the original one in terms of lowered maximum die stress, increased bearing pressure, and decreased distortion of the final products. The results of the extrusion experiments indicate that, with the optimized die, a highly qualified tube with good appearance, desired dimensional accuracy, fine-grained microstructure, and high-pressure bearing capacity is manufactured successfully, and the die failure is delayed effectively.

Flow boiling in horizontal multiport tube: Development of new heat transfer model for rectangular minichannels

The flow boiling characteristics of R32 and R1234ze(E) were investigated in a horizontal multiport tube with rectangular minichannels. The local heat transfer coefficient and frictional pressure drop were measured under mass fluxes of 50–400 kg m−2 s−1 at a saturation temperature of 15 °C. The heat transfer was characterized by forced convection in the region with a low heat flux, high mass flux, and high vapor quality; the heat transfer was characterized by nucleate boiling in the region with a high heat flux and a low vapor quality. Moreover, the heat transfer was characterized by the thin liquid film formed around the elongated vapor plugs with a low mass flux and low heat flux. These heat transfer mechanisms were also confirmed based on the visualization results of the flow boiling. The heat transfer deteriorated in the plug and slug-annular flows due to the occurrence of dry patches. A new heat transfer model for multiport rectangular minichannels was developed that considered forced convection, nucleate boiling, and thin liquid film evaporation. Moreover, this new heat transfer model considered flow patterns inside the multiport minichannels, heat transfer deterioration due to dry patches, and dryout inception quality. The newly model can predict the heat transfer characteristics in multiport rectangular minichannels for a mean deviation of less than approximately 15%. The frictional pressure drop of R32 was smaller than that of R1234ze(E) under the same mass flux and vapor quality conditions and compared with previous correlations.

Influence of the Manifold Configuration of Pulsating Heat Pipes on Their Performance

The performance characteristics of three air-to-air pulsating heat pipes with R1233zd as a working fluid for cooling an electronic cabinet are presented. Each of these pipes comprises two coolers staked in one unit of dimension 36 × 19 × 5 cm3, constructed with extruded multiport tubes and louvered fins brazed together. These pipes differ from each other only in their manifold configuration achieved by having fluid distribution plates at the evaporator and condenser manifolds to provide a serial connection of the flow paths. The experiments were performed with PHPs of only one type, composed of two coolers in stack and turned through 90° on their narrow side. A new parameter, representing the rate of pressure pulsations in a PHP, was introduced into consideration, and it is shown that this parameter correlates with the PHP performance.

Strength Variation in Processing Multiport Extrusion Tubes of A1100 and A3102 Alloys

The manufacturing process of multiport extrusion tubes generally includes homogenization, extrusion, roll leveling and heat treatment. In order to investigate the influence of the manufacturing procedures on strength variation of multiport extrusion tubes made of A1100 and A3102 alloys, the tube fabrication experiments and following materials characterization are carried out. The alloys’ stress–strain curves after every processing procedure are measured, and pressure-bearing capacity of the tubes is tested. The tubes’ strength and pressure-bearing capacity reach the peak values after straightening process and drop to the minimum after annealing. The contributions of solid solution hardening, grain boundary hardening and constituent particle strengthening to yield strength are evaluated. It is deduced that cluster hardening is the dominant strengthening mechanism for the as-homogenized and as-extruded samples; the contribution of clusters is second to work hardening after straightening process; slight cold work in conjunction with high-temperature annealing accelerates abnormal grain growth. The loss of strength increment from clusters can be interpreted by the remarkable reduction of high-angle grain boundaries after abnormal grain growth.

Developing Convective Heat Transfer in Multiport Microchannel Flat Tubes

An experimental investigation of fluid flow friction and heat transfer coefficient in simultaneously developing flow through a multiport microchannel flat tube (MMFT) was presented. The cross-sectional geometries of five tubes were rectangular with hydraulic diameters of 0.8–1.33 mm and aspect ratio of 0.44–0.94. The working fluid was water, and the Reynolds number was in the range 150–4500. The experiment result showed that friction factor was successfully predicted by classical correlation in laminar regime, whereas the laminar–turbulent transition in the developing flow was not as obvious as in the completely developed flow. The greater aspect ratio produced stronger heat transfer capacity in the developing flow, although the effect of the aspect ratio decreased at increased Reynolds numbers for heat transfer characteristics. Moreover, the scale effect improved the heat transfer performance of MMFTs, especially at high Reynolds numbers.

An experimental investigation of flow boiling heat transfer coefficient and pressure drop of R410A in various minichannel multiport tubes

The present work demonstrates the two-phase flow boiling heat transfer coefficient and pressure drop of R410A inside various minichannel multiport tubes. The experimental investigation was performed in four tube types with the hydraulic diameters of 1.16, 1.14, 1.07 and 0.96 mm and the number of parallel channels are 7, 11, 16 and 9, respectively. The data were conducted with the heat fluxes of 3 and 6 kW m−2, the mass flux ranging from 50 to 500 kg m−2 s−1, the saturation temperature of 6 °C and the vapor quality up to 1.0. The heat transfer coefficient of R410A was found to be affected by the heat flux, mass flux, vapor quality and the geometry of channel. The frictional pressure drop gradient was also affected by the mass flux and vapor quality but independent with the variation of heat flux. In addition, the present data were compared with various heat transfer coefficient correlations and pressure drop models in literature. Finally, new correlation of heat transfer coefficient was proposed based on the present experimental data.

Microstructure and mechanical properties development of micro channel tubes in extrusion, rolling and brazing

In addition to extrusion, the post-processing of rolling and brazing has significant effect on the mechanical properties of multi-port tubes. The aluminum micro channel tubes, especially the seam welding zone in the internal webs, are investigated in terms of mechanical tests, microstructure characterization, and finite element analysis. The effect of the post-process of rolling are quantified by varying amount of rolling reduction ratios. Tensile strength and pressure bearing capacity are determined by tensile tests and burst pressure tests, respectively. The evolution of the microstructure within the seam wielding zone is characterized by Electron backscattered diffraction (EBSD) and optical microscopy (OM). The fracture profile observed by OM indicates ductile fracture mode. The pressure bearing capacity is reduced by ~47% at 5% reduction ratio of rolling followed by 3-minute brazing, where largest grains are obtained due to abnormal grain growth.