Circular Minichannel Research Articles

Experimental and Numerical Analysis of Heat Transfer and Flow Phenomena in Taylor Flow Through a Straight Mini-Channel

Abstract This study experimentally and numerically investigates the hydrodynamic characteristics and heat transfer of developing and fully developed laminar liquid–liquid Taylor flows. The problem is conducted in circular mini-channels with different diameters subjected to a constant wall temperature boundary condition. An experimental setup is designed employing an open-loop water/oil two-phase nonboiling flow at mini-scale tubing sizes of 1.42, 1.52, and 1.65 mm. Two silicone oils with the dynamic viscosities of 1 and 5 cSt at several volumetric flow rates are used to establish segmented flow. The impacts of the channel diameter, viscosity, and flow rate ratio on the flow pattern, pressure drop, film thickness, and heat transfer rate are discussed. In good agreement with the literature, it is found that the pressure drop generated by the interface increases the total pressure loss by up to 200% compared to the single-phase flow. The results also explain how recirculating regions within the slugs influence the film region and the physics of backflow. Furthermore, introducing segmented water slugs significantly enhances the heat transfer rate as the dimensionless thermal length decreases. A significant relation between the recirculating regions and heat transfer has been demonstrated for the first time.

Condensation heat transfer coefficient during steam intermittent flows in a horizontal minichannel

Contents of the paper refer to experimental investigations and subsequent prediction of the heat transfer coefficient for condensation of steam flowing in the 2.13 mm i.d. horizontal smooth circular minichannel. Experimental data obtained cover comprehensive span of two-phase flow conditions such that total mass flux is in the range of 178 kg/(m2·s) ≤ G ≤ 885 kg/(m2·s), steam quality varies within 0.003 ≤ x ≤ 0.118 and saturation temperature is ranging between 120.2 °C ≤ Ts ≤ 151.8 °C. The flow patterns were observed visually and images of interest were recorded and subsequently identified based on a map from the literature. There are three basic two-phase flow patterns studied such as slug, slug-annular and churn occurring in the tested minichannel during intermittent flow. Measurements of the local peripherally averaged condensation heat transfer coefficient for each of the patterns considered were carried out using of a heat flux sensor.Based entirely on empirical results achieved the correlation equations specific to the patterns studied have been developed in terms of pertinent dimensionless numbers predicting values of the condensation heat transfer coefficient measured at deviation of ±15%. The correlations show that the condensation process studied combines contributions by gravity induced falling-film condensation with superimposed influence of the liquid phase axial flow, steam quality and interactions by capillarity and surface tension forces.Validity of the correlations proposed is presented in terms of variability ranges of the dimensionless numbers employed specific for each of the pattern studied such as slug, slug-annular and churn. The data predicted on the correlations developed are then compared against those computed on well recognized contributed relationships referenced to steam in flow condensation found in the literature and discrepancies between the predicted and contributed results are displayed and discussed. The comparison reveals some cases for which the results predicted on the correlations proposed can differ substantially from those computed on existing contributions particularly when referenced to intermittent flows at slug and churn patterns.

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%.

Flow Boiling Heat Transfer Intensification Due to Inner Surface Modification in Circular Mini-Channel

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange devices. We present experimental results of the surface modification effect on hydrodynamics and flow boiling heat transfer, including data on the CHF. A description of the experimental stand and method for modifying the test mini-channel is also presented. The studies were carried out with freon R-125 in a vertical mini-channel with a diameter of 1.1 mm and a length of 50 mm, in the range of mass flow rates from G = 200 to 1400 kg/(m2s) and reduced pressures between pr = p/pcr = 0.43 and 0.56. The maximum surface modification effect was achieved at a reduced pressure of pr = 0.43, the heat transfer coefficient increased up to 110%, and the CHF increased up to 22%.

Effects of curvature existence, adding of nanoparticles and changing the circular minichannel shape on behavior of two-phase laminar mixed convection of Ag/water nanofluid

In the present research, heat transfer behavior and Ag/water two-phase laminar nanofluid flow as cooling fluid in volume fraction of nanoparticles 0–6 % and Reynolds numbers of 200–500 in a tube with curvature angle of 270° are simulated utilizing finite volume method. 3-D simulations of laminar mixed convection two-phase nanofluid flow considering Grashof numbers of 5,000, 50,000 and 100,000 are carried out numerically. The results of the present research indicate that adding solid nanoparticles causes periodic behavior with particular repeat length on the axial velocity profile and maximum and minimum values shift every 60°. Also, after the curvature angle of 30° because of fluid motion, centrifugal forces importance, fluid rotation, and hydrodynamic field, local friction factor increases considerably up to channel outlet. In all diagrams, the maximum friction factor is related to the Grashof number of 100,000, this value will have more significant augmentation by the increase of volume fraction. Due to better fluid mixing and increase of temperature line slop at Reynolds number of 500, heat transfer enhances at the regions of the bottom bend of the channel. Higher volume fractions of nanoparticles reinforce the heat transfer mechanism and heat absorption from the hot surface. In the regions where fluid is influenced by thermal boundary layer growth, entropy generation is augmented. This entropy generation is due to heat transfer. On the other hand, entropy generation is reduced in the regions where the effect of inlet fluid temperature is dominated.

Condensation heat transfer of pure refrigerants R1234yf and R32 inside multiple circular minichannels

This study experimentally investigated condensation heat transfer coefficients inside horizontal multiple circular minichannels with inner diameters of 0.81 and 0.49 mm, using pure refrigerants R1234yf and R32. The effects of mass flux, vapor quality, channel size, heat flux, and refrigerant properties on condensation flow characteristics within a mass flux range of 50–400 kgm−2s−1 were clarified. Annular, churn, slug–annular, and plug (intermittent) flows were visualized inside multiple circular minichannels. It was determined that the heat transfer coefficients of R1234yf and R32 were affected by mass flux, vapor quality, and heat flux. At lower mass fluxes, the contribution of surface tension to heat transfer was dominant relative to that of forced convection. The heat transfer coefficient increased with decreasing channel size, and R32 exhibited higher values of the heat transfer coefficient compared to those exhibited by R1234yf. The proposed heat transfer correlation, accounting for the effects of forced convection and surface tension, predicted heat transfer coefficients for circular minichannels with a mean absolute deviation of 15.2%.

ANALISA NUMERIK PENINGKATAN TRANSFER KALOR ALIRAN TURBULEN FLUIDA NANO TiO₂/AIR PADA CIRCULAR MINICHANNEL

Conventional heat transfer fluids such as water, ethylene glycol and oil are limited by their poor thermal properties therefore it is needed the advanced heat transfer fluids that be able to improve their thermal performance. Nanofluids is a colloidal dispersion of nano-sized particles which are a breakthrough of thermal system. This numerical study using the multi-phase mixture model method investigated the convective heat transfer of TiO2/water nanofluids flowing through a circular mini channel with a 1.09 mm diameter and 306 mm length under turbulent flow regime. TiO2 nanoparticles with a diameter of 21 nm were dispersed into water with volume concentrations of 1.0, 2.0, 3.5, and 5.0 vol.%. The Reynolds number was varied from 4000-20,000 and a constant heat flux of 6500 W/m2. The results showed that the addition of nanoparticles and the variation of the Reynolds number increased the convective heat transfer coefficient of TiO2/water nanofluid by 5.18%, 7.4%, 12.4%, and 14.3%, respectively, with increasing in nanoparticles concentrations.

Flow boiling heat transfer, pressure drop and flow patterns of the environmentally friendly refrigerant R1234yf for cooling avionics

• Slug, throat-annular, wavy-annular, annular, and annular-mist flow are identified. • HTC and FPD are affected by mass flux, heat flux, saturation pressure and vapor quality. • Flow boiling HTC and FPD of R1234yf are smaller than that of R134a within 20%. • Nucleate and convective boiling dominate regions split by a vapor quality of 0.4. • New HTC and FPD correlations show mean absolute deviations of 9.3% and 6.3%. Flow boiling technology is an effective way to solve the heat dissipation problem of avionics. However, studies on the flow boiling characteristics of environmentally friendly refrigerants are still insufficient. To expand the related global database containing both experimental conditions and results, new experimental data are needed. Here, horizontal flow boiling experiments were conducted using R1234yf, and the heat transfer coefficients (HTCs), frictional pressure drops (FPDs) and flow patterns in a 1.88 mm circular minichannel were obtained under mass fluxes, saturation pressures and heat fluxes of 400–870 kg m −2 s −1 , 0.6–0.8 MPa and 40–65 kW m −2 , respectively. For comparison, experiments using R134a were carried out under the same conditions. The results show that for both R1234yf and R134a, the HTC increased with increasing mass flux, heat flux and saturation pressure, while the FPD decreased with increasing saturation pressure and increased with increasing mass flux but remained almost constant with heat flux. The heat transfer was dominated by nucleate boiling at lower vapor quality and transformed into convective boiling at higher vapor quality, with a threshold vapor quality of around 0.4. Typical flow patterns, including slug flow, throat-annular flow, wavy-annular flow, annular flow, and annular-mist flow, were identified through visualization. Through comparison, it was found that the HTC and FPD of R1234yf are smaller than those of R134a under the same conditions. In addition, based on the experimental HTC and FPD, several correlations yielding smaller predicted deviations are suggested, and new correlations having mean absolute deviations of 9.3 % and 6.3 % for flow boiling HTC and FPD of R1234yf are proposed.

Corrigendum to “Correlation between pressure loss and heat transfer coefficient in boiling flows in printed circuit heat exchangers with semicircular and circular mini-channels” [Appl. Therm. Eng. 204 (2022) 117963

Corrigendum to “Correlation between pressure loss and heat transfer coefficient in boiling flows in printed circuit heat exchangers with semicircular and circular mini-channels” [Appl. Therm. Eng. 204 (2022) 117963

Correlation between pressure loss and heat transfer coefficient in boiling flows in printed circuit heat exchangers with semicircular and circular mini-channels

• Boiling flow heat transfer coefficient and pressure drop were evaluated. • Printed circuit heat exchangers with semicircular and circular channels were employed. • Nucleate boiling in corners affected the heat transfer and flow resistance. • Heat transfer coefficient in semicircular channel was twice as high at low quality. • Increase in pressure drop was suppressed by effect of corners at a low quality. In this study, we investigated the correlation between the pressure loss and heat transfer coefficient for boiling flows in printed circuit heat exchangers. The effect of channel cross-sectional shapes—semicircle and circle—was evaluated. The hydraulic diameters of the semicircular and circular channels were 1.04 mm and 1.0 mm, respectively. A subcooled liquid with a subcooling temperature of 10 K was heated with hot water, and the saturation temperature was 30 °C. The refrigerant mass flux was set in a range of 100–400 kg/m 2 ·s. Consequently, despite the lower pressure drop, for low outlet quality, the heat transfer coefficient was higher for the semicircular channel than for the circular channel. The heat transfer was enhanced by the promotion of bubble nucleation in the thick liquid film at the corners, and the flow resistance was suppressed by the decrease in the vapor shear stress. However, for high outlet quality, the difference in the heat transfer coefficient decreased with an increase in the outlet quality. The experimental results were compared with the results obtained by correlation equations. The lowest mean absolute error of heat transfer coefficient and pressure drop for the semicircular mini-channel were 15.1 % and 26.7 %, respectively.

Flow Regimes and Transitions for Two-Phase Flow of R152a During Condensation in a Circular Minichannel

Two-phase flow regimes were experimentally investigated during the entire condensation process of refrigerant R152a in a circular glass minichannel. The inner and outer diameters of the test minichannel were 0.75 and 1.50 mm. The channel was 500 mm long to allow observation of all the two-phase flow regimes during the condensation process. The experiments used saturation temperatures from 30 to 50°C, a mass flux of 150 kg/(m2·s) and vapor qualities from 0 to 1. The annular, intermittent and bubbly flow regimes were observed for the experimental conditions in the study. The absence of the stratified flow regime shows that the gravitational effect is no longer dominant in the minichannel for these conditions. Vapor-liquid interfacial waves, liquid bridge formation and vapor core breakage were observed in the minichannel. Quantitative measurements of flow regime transition locations were carried out in the present study. The experiments also showed the effects of the saturation temperature and the cooling water mass flow rate on flow regime transitions. The results show that the annular flow range decreases and the intermittent and bubbly flow ranges change little with increasing saturation temperature. The cooling water mass flow rate ranging from 38.3 kg/h to 113.8 kg/h had little effect on the flow regime transitions.

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of a Low-GWP Refrigerant R1234ze(E) in a Horizontal Minichannel

To protect the environment, a new low-GWP refrigerant R1234ze(E) was created to substitute R134a. However, its flow boiling performances have not received sufficient attention so far, which hinders its popularization to some extent. In view of this, an experimental investigation was carried out in a 1.88 mm horizontal circular minichannel. The saturation pressures were maintained at 0.6 and 0.7 MPa, accompanied by mass flux within 540–870 kg/m2 s and heat flux within 25–65 kW/m2. For nucleate boiling, a larger heat flux brings about a larger heat transfer coefficient (HTC), while for convective boiling, the mass flux and vapor quality appear to take the lead role. The threshold vapor quality of different heat transfer mechanisms is around 0.4. Additionally, larger saturation pressure results in large HTC. As for the frictional pressure drop (FPD), it is positively influenced by mass flux and vapor quality, while negatively affected by saturation pressure, and the influence of heat flux is negligible. Furthermore, with the measured data, several existing correlations are compared. The results indicate that the correlations of Saitoh et al. (2007) and Müller-Steinhagen and Heck (1986) perform best on flow boiling HTC and FPD with mean absolute deviations of 5.4% and 10.9%.

An experimental study on optimization of $${\mathbf{SiO}}_{2}$$/water nanofluid flows in circular minichannels

An experimental study on optimization of $${\mathbf{SiO}}_{2}$$/water nanofluid flows in circular minichannels

Thermal and hydrodynamic analysis of a compact heat exchanger produced by additive manufacturing

The demand for higher heat transfer effectiveness has stimulated the combination of compact heat exchangers and additive manufacturing. The potential to fabricate complex geometries with different materials can optimize the trade-off between heat transfer and pressure drop. In this work, theoretical models for thermal and hydrodynamic performance were developed for a cross-flow compact heat exchanger manufactured with the SLM process, an alternative to Printed Circuits Heat Exchangers. The heat exchanger core has a cubic format with 100 mm edge and 2 mm channel diameter. The raw material used is AISI 316L stainless steel. Furthermore, the relative density of the prototype is 99.8% and the surface roughness measured was 12.21 µm. The one-dimensional steady-state with circular mini channels models are validated with experimental data. The experiments were evaluated in laminar, transition, and turbulent regimes, with different temperatures. The theoretical models have a good agreement, with 3.3% and 15.3% average errors for the thermal and hydrodynamic models. Besides, the impact of surface roughness in the pressure drop was negligible. The influence of replacing the core material in the thermal performance is significant in the turbulent regime.

Application of RQA and SOM for identification of two-phase flow patterns during boiling in horizontal minichannel

In the paper, numerical methods of data analysis recurrence quantification analysis (RQA) and self-organizing map (SOM) have been used to analyse pressure drop oscillations during the flow boiling in minichannel. The performed analysis allows us to identify flow patterns based on the character of the pressure drop oscillations. The following two-phase flow patterns have been identified: liquid flow, liquid flow with small vapour bubble, slug flow, long slug flow and confined bubble flow. In the experiment, the open-loop boiling system in a circular horizontal minichannel with an inner diameter of 1 mm was investigated. The two-phase flow patterns at the outlet of the heated section were observed through the glass tube (with an inner diameter of 1 mm) and recorded by a high-speed camera Phantom v1610.

Accelerating Taylor bubbles within circular capillary channels: Break-up mechanisms and regimes

In the present paper, an enhanced Volume of Fluid model is applied for the conduction of parametric numerical simulations, to investigate break-up phenomena of accelerating, elongated, vapour bubbles, within circular mini-channels. The effect of fundamental controlling parameters in the resulting break-up characteristics is investigated. Four different series of parametric numerical simulations of isolated vapour bubbles within mini-channels are performed, examining the effects of the imposed pressure difference between the inlet and the outlet of the channel, the surface tension, the effect of the applied heat flux as well as the initial liquid film thickness between the bubbles and the channel, on the developed vapour/liquid interface dynamics. The overall dimensionless number ranges examined are 6.76 < We < 1474.7, 0.007 < Ca < 0.14, 694 < Re < 12541 and by introducing a modified Froude number in order to account for the flow acceleration, 1 < Fr* < 21.86. These dimensionless number ranges are selected in order to overlap with experimental observations in zero-gravity Pulsating Heat Pipe experiments that constitute the motivation for the present numerical investigation. The proposed simulation results identify three prevailing regimes. A “full break-up” regime, a “partial break-up” regime and a “no break-up” regime. The entrainment of liquid droplets at the trailing edge of the vapour slugs is in most cases responsible for their subsequent “full break-up”, into a leading and a trailing bubble, as it is identified from the numerical simulations. Moreover, the applied heat flux does not influence the resulting break-up regimes. Finally, these identified break-up regimes, are grouped together into a well-defined flow map with respect to the We and Fr* numbers.

Condensation and flow boiling heat transfer of a HFO/HFC binary mixture inside a minichannel

Refrigerant blends obtained mixing hydrofluorocarbons (HFC) and hydrofluoroolefins (HFO) have recently been proposed as substitutes for high Global Warming Potential fluids employed in refrigeration and air-conditioning systems (e.g. R404A and R410A). Since these new mixtures have usually a non-azeotropic behavior, exhibiting a temperature variation during isobaric phase change, new heat transfer coefficients data are needed to assess available models or to develop new correlations. In the present paper, condensation and flow boiling heat transfer coefficients are measured inside a 0.96 mm diameter circular minichannel with a binary mixture of R32 and R1234ze(E) (0.75/0.25 by mass composition). Condensation heat transfer coefficients are measured at saturation pressure of 21.8 bar (corresponding to a dew point temperature of 41.5 °C) with mass flux ranging between 150 kg m−2 s−1 and 800 kg m−2 s−1. Flow boiling tests are run at 17 bar (corresponding to a bubble temperature of 28.7 °C), mass velocity between 300 and 600 kg m−2 s−1, and heat flux between 30 and 245 kW m−2. Mixture heat transfer coefficients are compared against data of pure fluids components and against data taken with R32/R1234ze(E) blends at varying compositions. The importance of the mass transfer resistance and the heat transfer penalization are discussed. Experimental data are used to assess correlations for the estimation of the condensation and flow boiling heat transfer coefficient.

Numerical study of the effects of nanorod aspect ratio on Poiseuille flow and convective heat transfer in a circular minichannel

The effects of nanorod aspect ratio on Poiseuille flow and convective heat transfer of a nanofluid were studied numerically. A coupled model was proposed that considered the non-uniformity of the nanoparticle volume fraction and the orientation distribution. The thermal properties of the base fluid varied with temperature, and the nanofluid viscosity and thermal conductivity were correlated with the particle volume fraction based on experimental data. The model was verified useful to predict nanofluid flow that contains nanorods by comparing the numerical results with experimental data. The non-uniformity of nanorod volume fraction distribution increases near the boundary when the aspect ratio is larger. The nanorods orient nearly randomly-in-space along the flow center and align around the flow direction when particles migrate towards the wall. The flow resistance increases with the augment of the nanorod aspect ratio. The radial velocity profile of the fully developed Poiseuille flow is flattened due to the non-uniformity of the apparent stress, and the effect is enhanced with the increase of the nanorod aspect ratios. The convective heat transfer and overall thermal performance of the nanofluids is enhanced with the augment of the nanorod aspect ratio, especially in the low-flow-rate region. Hence, non-spherical nanofluids can be applied to cooling applications in minichannels with low-power consumption.

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.

Water flow boiling heat transfer in vertical minichannel

In recent years, microchannel water flow boiling has demonstrated excellent heat removal capabilities, particular for nuclear reactor safety utilities, ranging from mini-scale cooling devices to chemical industry plant purposes. Of particular interest is cooling technology using water as the primary refrigerant. This paper presents the results of an experimental investigation into the water boiling heat transfer in a minichannel conduit vertically oriented with the upward flow. The test section was a circular minichannel tube made of stainless steel with a 2.12 mm inner diameter. To observe the flow regime, the test section was equipped with sight tubes made of transparent perfluoroalkoxy alkane resin and Pyrex glass at the inlet and outlet, respectively. Water flow boiling experiments were conducted at a saturation temperature of approximately 100 °C, with the mass flux of water as the working fluid from 20 to 120 kg/m2·s, and the average vapor quality in the test section inlet of approximately 0.05 to 0.9. The predicted two-phase flow regimes at the test section were classified as three patterns, namely slug, annular, and churn flows, using a flow pattern map with the vapor quality as the abscissa versus the mass velocity as the ordinate. The flow patterns were confirmed and visualized by observations at the test section outlet sight tube. The characteristics of the heat transfer and fluid flow were analyzed. Moreover, the forced convective heat transfer coefficient and pressure drop in the test section were determined based on the experimental data. The highest mass flux yielded the highest convective heat transfer coefficient. Despite this significant heat transfer coefficient improvement, the pressure loss was higher than that for a higher mass flux, indicating that the pressure loss was more pronounced owing to the presence of the minichannel at a higher mass flux.