Pressure Drop Characteristics Research Articles

Review on fabrication and experimental study of microchannel heat sinks for cooling of electronic components

Microchannels are widely used in electronic components for higher heat dissipation rate and for providing optimum performance and durability for electronic devices. In this paper, various aspects of microchannel including its fabrication and experimental study are being reviewed. fabrication of microchannels can be done using machining techniques like micro-milling, electric discharge machining, laser beam machining, lithography, injection molding, dry and wet etching. Experimental comparison of fluid flow and heat transfer characteristics in manufactured microchannel heat sinks utilising laminar water flow as a coolant in common applications. Also, coolants like hybrid and non-hybrid nanofluids, ethylene glycol, phase changing material, graphene, etc. are used to enhance the thermal conductivity and heat dissipation rate in microchannel heat sinks. Experimental studies on various output parameters like surface roughness, tool wear rate, and material removal rate are examined. Complex structured microchannel heat sinks are also considered. The variations in process parameters with respect to material removal rate and surface roughness can be found using an experimental study. Also, optimization of various process parameters along with material removal rate and surface quality can be performed using numerical analysis. Heat dissipation performance based on the geometry of the microchannel heat sink is explored. These aspects determine the pressure drop characteristics as well as fluid flow and heat transfer capability of a microchannel heat sink. It is inferred that fabrication, experimental and numerical factors have a remarkable influence on the fluid flow and heat dissipation performance of the microchannel heat sink.

Applying 3D X-ray Microscopy to Model Coated Gasoline Particulate Filters under Practical Driving Conditions.

Recent progress in 3D X-ray microscopy allows the analysis of coated gasoline particulate filters on a detailed pore-scale level. However, derivable detailed three-dimensional models for filter simulation are not applicable under transient driving conditions of automotive aftertreatment systems due to their inherent complexity. Here, we present a novel concept to utilize highly resolved 3D X-ray microscopy scans and their quantitative analysis for a macroscopic model of coated gasoline particulate filters intended to be applied in a driving cycle. A previously developed filtration model build on a 1D + 1D flow model on the channel scale of a filter is utilized. Accompanying measurements conducted on a dynamic engine test bench serve as validation for pressure drop and filtration characteristics. With the determined properties from 3D X-ray microscopy, the macroscopic model successfully replicates the measurements. Regarding the filter coating, the reduced porosity and a decrease of medium sized pores relative to an uncoated substrate reduce the filtration efficiency under steady-state as well as transient conditions.

A hydrophobic porous substrate-based vapor venting technique for mitigating flow boiling instabilities in microchannel heat sink

With the onset of nucleation, flow boiling in microchannels is susceptible to undesirable flow instabilities resulting in fluctuating wall temperature and large pressure drops. It is imperative to develop a passive technique to mitigate flow boiling instabilities and enhance the thermo-hydraulic performance of the heat sink without additional pressure drops. The present work explores a hydrophobic porous polydimethylsiloxane (PDMS) substrate-based vapor venting technique to mitigate flow boiling instabilities in microchannels. The hydrophobic porous substrate fitted at the channel exit manifold attracts the vapor plug, assists in vapor venting, and thus facilitates the easy evacuation of the bubbles from the channel. Flow boiling experiments are performed with deionized water in rectangular cross-section microchannels for a heat flux range of 10–260 W/cm2 and the coolant mass flux range of 206–335.49 kg/m2s. Heat transfer and pressure drop characteristics of the proposed configuration are compared with those of the conventional configuration. The proposed design suppresses the wall temperature oscillation and pressure oscillations. It produces a 32 % enhancement in heat transfer coefficient and a 63 % reduction in pressure drop compared to the conventional outlet configuration. The new configuration significantly reduces the bubble ebullition cycle, resulting in an efficient bubble evacuation from the channels. Further, frequent flushing of the vapor slug in the new design reduces the temperature fluctuations and enhances the heat transfer coefficient. The heat transfer augmentation is more pronounced at higher heat flux with more vapor generation rates.

Numerical study on pressure drop characteristics for turbulent flow in conical spiral tubes

The study of pressure drop characteristics in a curved tube is important as it has extensive application in coiled tube heat exchangers, hydraulic and pneumatic systems. The literature review suggests that the pressure drop characteristics at the coil level as well as at local level for the curved tubes with varying curvature i. e. conical spiral tubes require further investigations. The objective of the present work is to investigate the flow resistance characteristics of turbulent fluid flow in conical spiral tubes with uniform wall temperature heating with the numerical simulation approach using ANSYS Fluent. The conical spiral tubes with different curvature and cone taper angle ranging from 10° to 60° were modelled and simulated for the Reynolds number range of 104 to 105. The CFD results reveal that the total pressure drop and the local cross-flow pressure loss coefficient increased with Reynolds number, cone taper angle and curvature ratio. The results also show that the friction factor and the cross-flow coefficient increased significantly in the tube at a cone taper angle of 60° due to an enhancement in the secondary flow and hence the pressure loss. For the given curvature radius and tube diameter of the conical spiral tube there exists an optimum Reynolds number at which the dimensionless cross-flow pressure coefficient has an optimum value. A correlation to predict the friction factor as a function of Reynolds number and cone taper angle is proposed for the turbulent flow in conical spiral tubes. For the usual practically applicable curvature ratio of less than 0.1 in curved tubes, the pressure drop prediction using the proposed correlation is found to be within ±10% of the numerical simulation results.

Experimental Investigation on Heat Transfer and Pressure Drop Characteristics of Food Additive in Dimple Plate Heat Exchanger

The present study attempts to examine the heat transfer and pressure drop aspects of a 11-channel dimple plate heat exchanger with hot water as the hot fluid and sodium benzoate (food preservative) as the cold fluid. The outcome of the mass flow rate of hot and cold fluids on the convective heat transfer coefficient and overall heat transfer coefficient were investigated. Furthermore, the effect of Reynolds number on the pressure drop and the Nusselt number were observed. The experimental results demonstrated that when the mass flow rate of the cold fluid increases, so does the overall heat transfer coefficient and the convective heat transfer coefficient. Convective heat transfer coefficient, overall heat transfer coefficient, pressure drop and Nusselt number were increased when sodium benzoate concentration is varied (0.2, 0.4, 0.6% w/w). A correlation is obtained on the basis of experimental results to estimate the Nusselt number as a function of Reynolds and Prandtl number.

Numerical investigation on heat transfer and pressure drop characteristics of moisture separator reheater with porous media method

The geometry structure of the Moisture separator reheater is very complex, which makes it almost impossible to calculate the whole domain and analyze the physical characteristics. To get the three-dimensional physical property distribution of the moisture separator reheaters, the porous media method is used to simplify the geometry. The 1-dimensional and 3-dimensional coupling theory is adopted into the calculation, for the primary side inside the finned tubes is iterated as the one-dimensional distribution, and the secondary side outside the finned tubes are treated as three dimensional. The whole process of conducting the porous media methods is illustrated with the value. Both the calculated and experimental results of the pressure drop are about 10 kPa in the 1st and 2nd reheaters. The calculation results provide a design and safety evaluation of the moisture separator reheater, as the three-dimensional physical distribution is got through the porous media calculation. With different input parameters such like the arrangement of the finned tubes, the calculated results of heat transfer and pressure drop will be different, and the calculated results are helpful to the design and optimization of the finned tube reheaters. This will help design a set of finned tube parameters.

Experimental investigation of the pressure-drop characteristics of open-cell metal foam with air-gaps under non-Darcy flow regimes

Multi-layer metal foam with discrete air gaps between neighbored layers can have wide engineering applications, and its pressure drop characteristic is the crucial performance that needs to be predicted precisely. Experimental studies were carried out to investigate the influences of the air gaps and their layouts on the pressure drop characteristics of the multi-layer open-cell metal foam within a wide flow velocity range to develop efficient models for pressure drop predictions. The accumulated metal foam thickness and air gap distance were kept unchanged in the tests, and four sets of test sections with different air gap layouts were tested. The varying airflow speed across the metal foam was realized by adjusting the inlet pressure level of the test section to reveal the relations of the pressure drop versus the mass flow rate. Results indicate that in the multi-layer and multi-gap configurations, the Forchheimer model in the transition regime and bilinear model in the turbulent regime is still applicable for predicting the pressure drop; however, the insertion of air gaps between the metal foam slices leads to an increased pressure drop coefficient. Meanwhile, the first air gap leads to a higher pressure drop compared to the subsequent air gaps and shifts the flow regime transition point towards an even more minor flow rate operating point, while the subsequent air gaps present almost the same influences on the metal foam pressure drop characteristics and transition point. With the experimental study, two models for predicting the pressure drop characteristics of open-cell metal foam with and without air gaps in the transition and turbulent flow regimes with a broadened range are presented.

Experimental and Numerical Study of Pressure Drop Characteristics of Soybean Grain under Vertical Pressure

The vertical pressure lead to increased airflow resistance through the grain bulk, which affected the efficiency of ventilation and drying. The effects of vertical pressures at 50, 150, and 250 kPa on the pressure drop characteristics of soybeans were studied using experiment and numerical simulation. The random packing and different compression states for soybean packed beds were generated by the Discrete Element Method (DEM). The Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) were coupled to investigate the radial velocity and pressure drop of soybean bulk. The simulation results showed that the radial porosity had an oscillating distribution, and the radial average dimensionless velocity was consistent with the distribution trend of porosity. The increase in vertical pressure causes a decrease in porosity and an increase in local velocity. The PathFinder code was used as a supplementary method to calculate the pore path and pore characterization parameters, and the resistance coefficient term in the Forchheimer equation was determined. The compression of soybeans measured by the experiment mainly occurred within two hours after loading. The pressure drop of soybeans increased with the vertical pressure, with the average pressure drop at vertical pressures of 50, 150, and 250 kPa being 36%, 57%, and 92% higher than the uncompressed state (0 kPa). The pressure drop of soybeans calculated by the DEM-CFD method and the Forchheimer equation under different vertical pressures were in close agreement with the experimental results, and an average relative difference was found to be less than 10%. These results provide guidance for estimating the pressure drop of soybeans at different grain depths.

Development of design guidelines for fin-and-tube heat exchangers with low-GWP refrigerants

Regulatory changes worldwide are requiring air conditioning and heat pump equipment manufacturers to switch refrigerants to alternatives with reduced global warming potential (GWP). Design of air-to-refrigerant fin-and-tube heat exchangers (FTHXs) is critical to overall performance of this equipment and is not currently well understood with low-GWP refrigerants. This paper presents a simulation study exploring the impact of design changes for a FTHX used in an indoor residential evaporator originally designed for R410A. Simulation results are used to generate design guidelines for FTHXs using HFO refrigerants R1234yf and R1234ze(E), evaluated using cooling capacity, refrigerant-side pressure drop and ratio of the two as critical metrics. When evaluated at identical operational conditions, with fixed refrigerant outlet superheat, it was found that for a fixed slab size, the FTHX was most sensitive to number of circuits, followed by tube diameter, number of tubes in the FTHX, and fin density. For the low-GWP HFOs, circuitry modifications led to maximum increase in FTHX capacity of 20%, and maximum decrease in refrigerant-side pressure drop of 88.6%. Based on simulation results, it is concluded that additional coil face area is required to match both capacity and refrigerant pressure drop, compared with R410A, this is demonstrated with two proposed FTHX designs. One design prioritizes increased FTHX capacity, and the other a reduction in refrigerant pressure drop. All these results culminated in design guidelines for HFOs that suggest it will be challenging to achieve both the same capacity and pressure drop characteristics as R410A without modifications to FTHX size.

Numerical study of buoyant flows in straight and converging chimneys

We numerically investigate the buoyancy-induced laminar flow in an adiabatic vertical chimney with straight and converging walls. The study is done with air as the fluid for different radii of the chimney, R = 15–50 mm, aspect ratios, L/R = 5–25, and inlet temperature, Ti = 350–550 K. In addition to these parameters, the converging chimney analysis are performed with different converging angles, θ = 1–5 degrees. The commercial CFD code Fluent is used to solve the equations governing the flow taking temperature effects on fluid properties into consideration. The effect of the above parameters on the buoyancy-induced mass flow rate has been presented. We see that the mass flow rate increases with an increase in Ti due to increased buoyancy with both straight and converging chimneys. With an increase in L/R, the mass flow rate increases for straight chimney and this is due to the frictional pressure drop characteristics of the chimney. On the other hand, the converging chimney shows a varying trend with L/R depending on the converging angle: with the increase in L/R the mass flow rate initially increases and then decreases for lower θ (<1o) and continuously decreases for higher θ (>1 o). The varying combined effect of the frictional and acceleration pressure drop characteristics of the chimney causes this change in trend. For the given inlet conditions and aspect ratio, the straight chimney would drive the highest mass flow rate. Based on the simulated data, we present correlations to predict the induced mass flow rate. For generality, we present correlations for the non-dimensional mass flow rate in terms of the relevant non-dimensional parameters i.e., the modified Gr number, the temperature ratio, and the area ratio. The correlations we present are applicable over a range that can be useful in designing chimneys for bio-mass gasification, household utilities, and medium-scale industries.

The pressure drop characteristics of humid air outside staggered tube bundles in the evaporative cooling process

Based on Euler-Lagrange method, a numerical model was established for the pressure drop characteristics of humid air outside staggered tube bundles (STB) during evaporative cooling in this paper. The influences of structural and operation parameters were analyzed for that. The results show that the pressure drop of humid air increases with the increase of mass flow rate of humid air and spray water, but it decreases with the increase of transverse and longitudinal distance. Meanwhile, the total pressure drop of humid air increases with the increase of tube diameter. It can also be found that the pressure drop in the transition area at the top of STB will increase with the STB height when the transverse distance is less than the minimum value (30 mm). Lastly, a new pressure drop correlation was proposed according to the CFD data. The error is less than 5% between the CFD results and the correlation results. The above conclusions will provide a theoretical basis for the research and selection of fan power in evaporative cooling heat exchangers.

Experimental Investigation of Two-Phase Flow Boiling Heat Transfer Coefficient and Pressure Drop of R448A inside Multiport Mini-Channel Tube

Regulations and restrictions against high global warming potential (GWP) refrigerants have been introduced to encourage the adoption of environmentally friendly refrigerants and mitigate the environmental impact of the HVAC industry. R448A, a zeotropic blend with a GWP of 1390, has recently been proposed as a drop-in replacement for R404A and R410A in commercial systems. In this study, the heat transfer coefficient and pressure drop characteristics of R448A within a multiport mini-channel tube were experimentally investigated. The experimental ranges of the mass and heat fluxes were 100 to 500 kg/(m2s) and 3–15 kW/m2, respectively. Additionally, the range of quality from 0 to 1 was considered at two fixed saturated temperatures of 3 and 6 °C. The heat transfer coefficient increased with mass flux. Under low mass flux condition, the heat flux increased the heat transfer coefficient, but there was no noticeable effect of the saturated temperature on the heat transfer coefficient. At high mass flux, heat flux had no major effect on heat transfer, while a decrease in the saturated temperature was found to increase the heat transfer coefficient. Moreover, the pressure drop increased with an increase in the mass flux and vapor quality, whereas the heat flux did not affect the pressure drop. The heat transfer coefficient and pressure drop performance of R448A was compared with that of R410A inside the same tube. Finally, correlations for heat transfer coefficient and pressure drop were proposed for the prediction of heat transfer coefficient and pressure drop in practical applications.

Pressure-drop characteristics of CO2 boiling flow in the regenerative-cooling channel of an Mg/CO2 powder rocket engine for Mars missions

To solve the problem of the thermal protection of Mg/CO2 powder rocket engines, how different working conditions in the regenerative-cooling channel influence the pressure drop of CO2 coolant is investigated. It is found that the proportion of two-phase acceleration pressure drop increases with increasing CO2 mass flux, inlet temperature, and heat flux and decreases with increasing backpressure. The ratio of acceleration pressure drop to total pressure drop is ∼0.3. With increasing mass flux, incoming flow temperature, and heat flux, the two-phase frictional pressure drop increases, and the increase of backpressure decreases the two-phase frictional pressure gradient. The factor flp is introduced to correct for the local pressure drop of the experimental system, and flp = 1.9819 when the two-phase coolant Reynolds number is between 20 000 and 50 000. For the unidirectionally heated cooling channel, the coolant is heated unevenly therein, so the heat-transfer mode has an important influence on the frictional pressure gradient of the coolant. It is found that μl can be used to predict the frictional pressure drop of μtp, but the deviation is still more than 20%. Based on experimental data, a new correlation for predicting the two-phase frictional pressure gradient is proposed. The relative deviation of the predicted data is only −7.02%, and 96.12% of the predicted data fall within the ±15% deviation band, so the prediction accuracy is greatly improved. The present research provides a theoretical basis for designing a regenerative-cooling scheme for an Mg/CO2 powder rocket engine.

Heat transfer and pressure drop characteristics of R1234yf during evaporation in a plate heat exchanger with offset strip fins: An experimental study

Plate heat exchangers (PHEs) have been suggested for use in secondary loop air conditioning systems in automobiles operated with flammable refrigerants, such as R1234yf. In this study, the evaporation heat transfer coefficient (HTC) and two-phase frictional pressure drop (FPD) of R1234yf in a brazed PHE with offset strip fins were investigated. The mass flux in the experiment was 40–80 kg m−2 s−1, heat flux was 4–10 kW m−2, vapor quality at the inlet was 0.1–0.8, and saturation temperature was 5–15°C. Prior to the primary analysis, a coolant-to-coolant test was performed using a modified Wilson plot approach to estimate the multiplier (C) and the Reynolds number exponential (n). The observed C and n values were 0.868 and -0.496, respectively, with an R2 value of 0.977. The main results demonstrated an increase in the evaporation HTC with an increase in vapor quality in the initial stages, followed by a subsequent decrease, forming a peak at a mean vapor quality of 0.35–0.41. The nucleate and convective heat transfer regimes were dominant during the evaporation of R1234yf at lower and higher mean vapor qualities, respectively. Evidently, the evaporation HTC was significantly influenced by the mass flux and vapor quality compared with the heat flux and saturation temperature. Furthermore, the two-phase FPD continuously increased with the mass flux, vapor quality, and heat flux, although it decreased with the saturation temperature. As a result, new correlations are proposed herein for forecasting the Nusselt number and friction factor of R1234yf with average absolute errors of ±12% and ±15%, respectively.

Modeling and Analysis of the Flow Characteristics of Liquid Hydrogen in a Pipe Suffering from External Transient Impact

Pipes can be subjected to external transient impacts such as accidental collision, which affects the safe operation of storage and transportation systems for liquid hydrogen. Fluid–structure coupling calculation for a pipe under external transient impact is performed, and the flow characteristics of liquid hydrogen in the pipe are analyzed. The pipe deforms and vibrates when suffering from external transient impact. Liquid hydrogen pressure in a cross-section plane increases along the pipe deformation direction. Additionally, external transient impact enhances the disturbance of liquid hydrogen near the pipe wall. The increased flow resistance and the energy induced by the deformed pipe both affect the flow of liquid hydrogen, and contribute to the fluctuated characteristics of liquid pressure drop. In addition, the phase state of liquid hydrogen remains unchanged in the pipe, indicating that little of the induced energy is transformed into the internal energy of liquid hydrogen. The work provides theoretical guidance for the safe operation of liquid hydrogen storage and transportation systems.

Experimental study on heat transfer and pressure drop characteristics in a microchannel heat exchanger assembly with S-shaped fins

Experimental studies on the water-to-water heat transfer and flow resistance characteristics of a microchannel heat exchanger assembly with S-shaped fins were conducted using the high-temperature and high-pressure water open experimental loop at Shanghai Jiao Tong University. The effects of different Reynold numbers of both the hot and cold sides on the heat transfer and flow resistance characteristics were analyzed. Correlations for the Nu number and friction factor for both the hot and cold sides were gained based on the experimental data. The same Nusselt correlation was used for both the hot and cold sides, and the fitting results showed satisfactory accuracy, where the fitting error was within ±15%. The power-law scheme was used to fit the total friction factor correlations, and the fitting results showed a fitting error of ±15% and ±12% for the hot and the cold sides, respectively. The correlations proposed in this study can be used in the design of this type of heat exchanger.

Heat transfer and pressure drop characteristics of heat exchangers based on triply periodic minimal and periodic nodal surfaces

The popularity of additive manufacturing has increased interest in the use of triply periodic minimal surfaces (TPMS) in engineering applications due to their potential for superior mechanical, heat and mass transfer properties. Periodic nodal surfaces (PNS) are a class of periodic continuous surfaces that also divide the space into non-intersecting, smooth and continuous domains like TPMS and can potentially have superior mechanical, heat and mass transfer properties. To evaluate the potential for superior performance, in this manuscript we characterize the flow and heat transfer properties of seven TPMS and PNS based structures by numerically computing the friction factors and Nusselt numbers in the laminar flow regime. This is the first study to evaluate the use of PNS as heat exchangers. In addition, it adds to the limited quantitative data available on the flow and thermal properties of TPMS. The friction factors associated with most of the TPMS and PNS based structures in this study are about an order of magnitude higher than that for laminar flow in tubes. These structures also had higher Nusselt numbers compared to tubes, with the enhancement increasing with increase in Reynolds number. Among the TPMS and PNS studied, Schwarz-D had the best heat transfer performance while Schwarz-P was the poorest performing structure. Basic heat exchanger design calculations showed that to remove the same amount of heat and operate under the same pressure conditions, the Schwarz-D based heat exchanger was 3–10 times smaller than a tubular heat exchanger. This shows how TPMS or PNS can be used to design heat exchangers with superior performance, especially in applications where space and weight are at a premium. • First demonstration of the use of non-TPMS Periodic Nodal Surfaces as heat exchanger. • Under laminar flow most TPMS/PNS had friction factors 3–10 times higher than pipes. • Under laminar flow most TPMS/PNS had Nusselt numbers 1–15 times higher than pipes. • TPMS/PNS heat exchangers can be significantly smaller than tubular heat exchangers.

Experimental study of ultralow flow resistance fractal microchannel heat sinks for electronics cooling

Silicon-based fractal tree-shaped microchannel heat sinks with different bifurcation angles were designed and manufactured. The heat transfer and pressure drop characteristics of the fractal microchannel network were experimentally studied under an inlet Reynolds number of Re0 = 500–1700. The results showed that compared with the traditional linear microchannel heat sink, the fractal microchannel heat sinks could achieve considerable flow drag reduction and heat transfer enhancement. Maintaining the chip temperature below 85 °C, the pressure drop can be reduced by up to 92%, and the heat transfer coefficient can be enhanced by 1.5 times of the fractal microchannel network at the same flow rate. The optimized bifurcation angle of the fractal network was recommended to be 30°. The COP of fractal microchannel heat sinks with a bifurcation angle of 30° can reach 12–13 times that of the linear microchannel.

Simulation of a solid-phase oxygen control scheme in lead-bismuth eutectic system

It is known that solid-phase oxygen control scheme is one of the important methods to solve the corrosion problem when we choose lead–bismuth eutectic (LBE) as coolant in lead-cooled fast reactor (LFR) or Accelerator Driven System (ADS). The mass-exchanger (MX), a porous chamber filled with mechanically stable lead oxide (PbO spheroids), is the key component of solid-phase oxygen control. In order to study the characteristics of oxygen mass transfer, pressure drop, flow field and oxygen concentration field in MX, a randomly packed bed model is established using DEM method. The fluid domain is meshed with computational fluid dynamics (CFD) code STAR-CCM +. The particle–particle and wall–particle contact points are modified with short cylinder bridges to improve the cell qualities. The mass transfer boundary layer theory is applied to explain the oxygen supply process of PbO sphere dissolution to LBE. The results of the average porosity, radial porosity distribution and pressure drop match the previous literature pretty well. The CFD results related to Sherwood number (Sh) are validated by CRAFT experimental data with an acceptable error. The law of oxygen transfer and distribution is obtained by studying the CFD contours and the spatial distribution of oxygen concentration and velocity data. The temperature dependence and velocity dependence of Sherwood number are obtained through multiple linear regression processing of CFD data. Finally, the effects of different temperatures and mass flow rates on oxygen concentration and pressure drop are evaluated to determine the suitable operating domain for MX.

Experimental research on refrigerant condensation heat transfer and pressure drop characteristics in the horizontal microfin tubes

The condensation heat transfer characteristics and pressure drop for R410A were investigated in the horizontal smooth and microfin tubes. The 6.35-mm-OD, 7-mm-OD and 8-mm-OD tubes with active length 2 m were selected for condensation test cooled by the heat transfer fluid (water) circulated in a surrounding annulus. A refrigerant superheating 2–3 °C was set at the test section inlet and the mass flux range was 300–1100 kg·m−2·s−1. The heat transfer coefficient (HTC) of the microfin tube was about 1.63–2.29 times of that of the smooth one. The microfin one yields ~28.9% higher pressure drop than the smooth one. Large helix angle and fin height were beneficial to enhance heat transfer due to the centrifugal force and surface tension, but increased the pressure drop. 6.35-mm-OD tube produced both the higher HTC and pressure drop than the 7-mm-OD and 8-mm-OD tubes due to the increased effect of the surface tension and the sheer stress at the gas-liquid interface. It was necessary to optimize finned structure to enhance heat transfer. Experimental data were also compared with previous correlations proposed for smooth and microfin tube. It was necessary to collect more data source and make a generally-accepted and accurate correlation in future.