Vertical Minichannel Research Articles

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.

Experimental investigation of nitrogen flow boiling heat transfer in a single mini-channel

Flow boiling heat transfer of nitrogen at high subcritical pressure conditions in a single vertical mini-channel with the diameter of 2.0 mm was experimentally investigated. The tested mass flux varied from 530 to 830 kg/(m2·s), the inlet pressure ranged from 630 to 1080 kPa, and the heat flux ranged from 0 to 223.2 kW/m2. Effects of the mass flux and the inlet pressure on the nitrogen boiling curve were examined. Results showed that within the limited test conditions, the merging of three boiling curves indicates the dominance of nucleate boiling and the inlet pressure has a positive enhancement on heat transfer performance. Three heat transfer trends were identified with increasing heat flux. At low heat fluxes, the heat transfer coefficient increases first and then decreases with vapour quality. At intermediate heat fluxes, the heat transfer coefficient versus the vapour quality presents an inverted “U” shape. At high heat fluxes, a double valley shape was observed and the partial dry-out in intermittent flow and annular flow helps to interpret the phenomenon. The increasing inlet pressure increases the heat transfer coefficient over a wide range of vapour quality until the partial dry-out inception. The lower surface tension and lower latent heat of evaporation enhance the nucleate boiling for higher inlet pressure. A modified experimental correlation (mean absolute error (MAE)=19.3%) was proposed on the basis of the Tran correlation considering both the nucleate boiling and the partial dry-out heat transfer mechanism.

Influence of Direct and Co-Flow Pre-Injection of a Gas Phase on Gas–Liquid Two-Phase Flow through a Vertical Minichannel

The influence of pre-injection of a gas phase on gas–liquid two-phase flow through a vertical minichannel is experimentally investigated. Circular minichannel of 2 mm inner diameter is considered for the investigations. Air is used as a gas phase and water is used as a liquid phase. The superficial gas and liquid velocities are kept as 0.0531–0.2653 and 0.1062–0.2653 m/s, respectively. Inlet premixing of gas–liquid two-phase flow is achieved with an injector equipped with a nozzle to inject a gas phase. Three nozzles are developed with inner diameters of 0.7, 2, 2.5 mm. Direct injection and co-flow pre-injections of gas phase are obtained with various positions of a nozzle (0, 25 and 50 mm) in an injector. Flow patterns such as bubbly flow, Taylor bubble flow and slug-annular flow are observed and captured using an EXILIM high-speed camera. The influence of nozzle size and its positions in an injector on the hydrodynamics of Taylor bubble flow as well as two-phase pressure drop are analysed in detail. Inlet premixing and two-phase flow hydrodynamics have significant influence of gas-phase pre-injection in a vertical minichannel.

Characteristics of natural convective heat transfer in a vertical minichannel with two grooves

Characteristics of natural convective heat transfer in a vertical minichannel with two grooves

Time-dependent study of boiling heat transfer coefficient in a vertical minichannel

Purpose The purpose of this paper is to determine the time-dependent heat transfer coefficient during FC-72 flow boiling in a 1.7-mm-deep vertical and asymmetrically heated minichannel. Design/methodology/approach The temperature of the minichannel heated wall was recorded continuously with the use of thermocouples. The heat transfer coefficients for the subcooled and saturated boiling regions at the heated wall–fluid contact surface were calculated from the Robin boundary condition. Both the wall and fluid temperatures were obtained from the solution of the inverse nonstationary problems in two adjacent domains: the heated wall and flowing fluid. The FEM with Trefftz-type basis functions was applied to solve the inverse problem. Findings The obtained time-dependent heat transfer coefficient in subcooled boiling achieved rather low values, whereas in saturated boiling, the coefficient was the highest at the channel inlet. The boiling curves were plotted to illustrate the results. Practical implications The results of experiments are the best source of information for the design of minichannel cooling systems used for thermoregulation of components and heat exchangers. High-tech minichannel heat exchangers are applied in various industrial applications as microelectronics devices, gas turbines, internal combustion engines, nuclear reactors, X-ray sources and organic rankine cycle (ORC) modules. Originality/value In the study, the Trefftz functions for the nonstationary Fourier–Kirchhoff equation with the factor describing void fraction were determined and then used to construct the time-dependent basis functions in FEM.

Condensation of propane in vertical minichannels

Experiments and models for in-tube pressure drop and heat transfer characteristics during condensation of propane at low mass fluxes (G = 75–150 kg m−2 s−1) in vertical minichannels (D = 1.93 mm) for two saturation temperatures (Tsat = 47 °C and 74 °C) are presented. Trends in heat transfer coefficients and frictional pressure gradients with mass flux and quality are reported and discussed. For the conditions under consideration in this study, heat transfer coefficient appears to increase with an increase in saturation temperature, while the frictional pressure gradient trend becomes relatively insensitive to an increase in saturation temperature. No correlation from the literature adequately predicted the results from the present study. Two models developed here for frictional pressure drop and condensation heat transfer coefficient demonstrate significant improvements in predicting the observed trends.

Effects of pin and wire electrodes on flow boiling heat transfer enhancement in a vertical minichannel heat sink

In this paper, electrohydrodynamic (EHD) enhancement of flow boiling heat transfer in a vertical minichannel heat sink was investigated experimentally. The pin and wire electrodes are used to create different kinds of non-uniform electric fields. Flow boiling experiments of refrigerant R-141b under electric fields are conducted under various applied voltages. The results show that the application of pin and wire electrodes can promote the Onset of Nucleate Boiling (ONB) and effectively enhance heat transfer coefficient. For the pin and wire electrodes, the EHD enhancement effect increases with the increase of applied voltage. At the same heat flux, heat transfer enhancement for pin electrode is higher than that for wire electrode at low applied voltage (U ≤ 400 V), but is lower than that for wire electrode at high applied voltage (U ≥ 550 V). Moreover, the obtained maximum enhancement ratio for pin and wire electrodes could reach 1.72 and 1.80, respectively. The influence area of pin electrode is limited, whereas the influence area of wire electrode is the entire electrode length at U ≥ 400 V. Based on the simulation and experiment results, the enhancing mechanism of EHD on flow boiling heat transfer in minichannel was discussed.

High pressure refrigerants condensation in vertical pipe minichannels

The paper consists a description of thermal and flow experimental research on high-pressure refrigerants condensation process in vertical minichannels. The influence of mass flux density, heat flux density, the vapour quality and minichannel's internal diameter on the condensation process was investigated. Under the same conditions, R404A, R407C, R410A refrigerants were condensed. The study also consists a summary of data obtained during condensation process of all three investigated refrigerants. The results of experimental studies were compared with the results of theoretical modeling according to the dependencies developed by other authors. The compliance of the modeling data with the results of the experiment was considered unsatisfactory. Based on the results of experimental data, heat transfer coefficient and pressure drop prediction models for low heat fluxes (q up to 5000 W/m2) were obtained.

Comparison of the 1D and 2D calculation models used for determination of the heat transfer coefficient during flow boiling heat transfer in a minichannel

The paper discusses the results of the flow boiling heat transfer in a vertical minichannel with rectangular cross-section. The heating element for FC-72 flowing in the minichannel is a thin plate. Infrared thermography is used to determine changes in the temperature on its outer side. The aim of thecalculation is to determine the heat transfer coefficient using 1D and 2D calculation models. Local values of heat transfer coefficient on the surface between the heated plate and boiling fluid are calculated from the Newton`s and Fourier`s laws. In 2D model the plate temperature distribution is obtained by solving the inverse heat conduction problem. The governing equation is solved by means of two methods: the non-continuous Trefftz method and the Beck method. The results are presented as plate temperature and heat transfer coefficient calculated using 1D and 2D models as a function of the distance fromthe minichannel inlet. The analysis of the results revealed that the values and distributions of the heat transfer coefficient calculated by means of both models were similar. This suggests that all mentioned methods are interchangeable.

Trefftz method in an inverse problem of two-phase flow boiling in a minichannel

The paper discusses the application of the Trefftz method for solving a coupled inverse problem concerning heat conduction and flow boiling. The T-complete functions were derived for a two-dimensional stationary energy equation in a flowing fluid, with a known velocity profile. The properties of these functions are described, with a focus on their relationship to harmonic polynomials. In view of the problem domain (two adjacent regions with different physical parameters), two subsequent inverse problems were formulated. The former was solved by approximating the temperature with a combination of harmonic polynomials while for the latter problem a twin-track approach was taken. The first method employed was the Trefftz method with newly generated functions. For comparison, the calculation was repeated using the Picard iterative process combined with the Trefftz method based on harmonic functions. The numerical calculations were conducted using experimental data concerning flow boiling of a refrigerant through a rectangular vertical minichannel. The computation resulted in obtaining two-dimensional temperature distributions (in each region) and a heat transfer coefficient at the solid-fluid interface versus the distance from the minichannel inlet. Both numerical approaches gave comparable results. The experimental heat transfer coefficient was compared with some correlations known from the literature.

Experimental study on flow boiling characteristics in a high aspect ratio vertical rectangular mini-channel under low heat and mass flux

This study investigates the flow patterns and flow boiling characteristics of R600a, R227ea and R245fa in a visualized vertical rectangular mini-channel, with a moderate-high aspect ratio (width/height = 6.25, cross-section area of 1.6 mm × 10 mm and length of 520 mm. The experimental results are obtained at saturation temperatures of 27.5, 36.6 and 45.5 °C, with relatively low heat and mass flux ranging from 3.60 to 10.50 kW/m2 and 32.20 to 116.8 kg/m2 s, respectively. Five flow types are identified and a flow pattern map is plotted. Front waves are observed and minimized bubbles are detected, generated from the slug to the churn-annular liquid film. The effects of heat flux, mass flux, vapor quality and saturation pressure on flow boiling are investigated and analysed. For all three tested refrigerants, the heat flux and saturation pressure are found to have a significant influence on the heat transfer coefficient, while the effects of the mass flux and vapor quality are negligible. Eleven existing correlations are comparatively evaluated based on the experimental data. The dimension groups correlation exhibits superior predicting performance, and a new correlation is developed for low heat and fluxes flow boiling in vertical minichannel.

Effects of electric field on flow boiling heat transfer in a vertical minichannel heat sink

Effects of applied DC electric field on flow boiling heat transfer in a vertical minichannel heat sink for two mass velocities and different heat fluxes are investigated experimentally. R141b is employed as working fluid. The heat sink contains eight rectangular minichannels, as well as both width and height of a single minichannel are 2 mm. Wire electrode is used and placed in the center of minichannel to create a non-uniform electric field. Flow visualization is performed using a high-speed video camera to analyze the interfacial features with and without electric field. Furthermore, the mechanism of two-phase flow boiling heat transfer enhancement under an electric field is explored. The results show that the onset of nucleation boiling slightly shifts towards downstream under electric field conditions. Electric field brings more vapor production, leading to early transition from bubbly flow to slug flow and early transition from slug flow to churn or annular flow. Electric field causes the bubbles to oscillate between the wall and the wire electrode, or renders the bubbles intermittently to touch the heated wall in the bubbly flow region. In the slug flow region, the surface of the elongated bubble intermittently touches the heated wall, and the interfacial shape irregularly changes with the bubble growing under electric field conditions. Overall, applying electric field can enhance the flow boiling heat transfer performance in the minichannel heat sink, and the enhancement effect is more significant in the bubbly and slug flow region. Moreover, the maximum enhancement ratio of 2.48 is obtained under present experimental conditions.

Experimental Investigation of the Characteristics of Flow Boiling Oscillations in a Vertical Minichannel

An experimental investigation is carried out to study the characteristics of flow boiling oscillation in a vertical minichannel with an inner diameter of 2.15 mm. The working fluid is deionized water at a constant system pressure of 0.101 MPa. The characteristics of flow oscillation with different working conditions are analyzed in this paper. With other constant working conditions, it is found that the oscillation periods of the pressure drop and the wall temperature decrease when the inlet subcooling decreases, while the fluctuation amplitude of pressure drop oscillations changes a little. The effects of heat flux and mass flux on the flow boiling oscillation are discussed with the equilibrium vapor quality (xe). Meanwhile, the xe which is at the onset of flow oscillation is affected by the mass flux. Finally, a new model which is based on the flow instability ratio (FIR) is proposed to predict the onset of boiling a two-phase flow oscillation, and a good agreement is achieved when comparing with the ex...

New cooling approach using successive evaporation and condensation of a liquid film inside a vertical mini-channel

In this paper, the successive evaporation and condensation of water-ethanol and water-methanol liquid film mixtures for cooling purpose is investigated. The simulations are achieved using a finite difference model to describe the heat and mass transfer during this process. The studied process relies on the absorption of energy at the evaporator and then the release of this energy at the condenser. The evaporation and heat exchange abilities of the mixtures composition are analysed to identify the appropriate mixture for the new proposed cooling approach. The results indicate that more energy equilibrium occurs with high concentration of water in the mixture with the advantage of having low bulk and interface temperature.

Using artificial intelligence to improve identification of nanofluid gas–liquid two-phase flow pattern in mini-channel

This work combines fuzzy logic and a support vector machine (SVM) with a principal component analysis (PCA) to create an artificial-intelligence system that identifies nanofluid gas-liquid two-phase flow states in a vertical mini-channel. Flow-pattern recognition requires finding the operational details of the process and doing computer simulations and image processing can be used to automate the description of flow patterns in nanofluid gas-liquid two-phase flow. This work uses fuzzy logic and a SVM with PCA to improve the accuracy with which the flow pattern of a nanofluid gas-liquid two-phase flow is identified. To acquire images of nanofluid gas-liquid two-phase flow patterns of flow boiling, a high-speed digital camera was used to record four different types of flow-pattern images, namely annular flow, bubbly flow, churn flow, and slug flow. The textural features extracted by processing the images of nanofluid gas–liquid two-phase flow patterns are used as inputs to various identification schemes such as fuzzy logic, SVM, and SVM with PCA to identify the type of flow pattern. The results indicate that the SVM with reduced characteristics of PCA provides the best identification accuracy and requires less calculation time than the other two schemes. The data reported herein should be very useful for the design and operation of industrial applications.

The heat transfer coefficient determination with the use of the Beck-Trefftz method in flow boiling in a minichannel

In this paper, the solution of the two-dimensional inverse heat transfer problem with the use of the Beck method coupled with the Trefftz method is proposed. This method was applied for solving an inverse heat conduction problem. The aim of the calculation was to determine the boiling heat transfer coefficient on the basis of temperature measurements taken by infrared thermography. The experimental data of flow boiling heat transfer in a single vertical minichannel of 1.7 mm depth, heated asymmetrically, were used in calculations. The heating element for two refrigerants (FC-72 and HFE-7100, 3M) flowing in the minichannel was the plate enhanced on the side contacting with the fluid. The analysis of the results was performed on the basis of experimental series obtained for the same heat flux and two different mass flow velocities. The results were presented as infrared thermographs, heated wall temperature and heat transfer coefficient as a function of the distance from the minichannel inlet. The results was discussed for the subcooled and saturated boiling regions separately.

Heat transfer coefficient determination using the FEM with time-dependent Trefftz-type basis functions in subcooled flow boiling in a minichannel

Results concerning flow boiling heat transfer in a vertical minichannel of 1.7 mm depth were shown. The channel was asymmetrically heated by a thin foil. Its surface temperature was recorded continuously in points by thermocouples. Measurements were carried out in 0.01 s intervals. The objective of the numerical calculations was to determine the heat transfer coefficient on the heated foil–fluid contact surface in the minichannel from the Robin boundary condition. Both the foil and fluid temperatures were the result of solving the nonstationary two-dimensional problem in the foil and flowing fluid. The problem was solved by using the FEM combined with Trefftz-type basis functions. The values of the time-dependent local heat transfer coefficient were presented and discussed.

Trefftz method in solving Fourier-Kirchhoff equation for two-phase flow boiling in a vertical rectangular minichannel

This paper presents the results of investigations into flow boiling heat transfer in an asymmetrically heated vertical minichannel of 1.7 mm depth. The heated element for FC-72 flowing in the minichannel was an alloy plate 0.45 mm thick, microstructured on one side, in direct contact with the flowing fluid. The computational part of the study contains approximate steady state solutions of the heat transfer problems described by Poisson.s equation and the energy equation for the heated plate and the fluid, respectively. For both equations, the boundary conditions were specified on the basis of experimental data. Temperature of the outer plate surface, measured by infrared thermography, and heat losses to ambient air were included in the calculations. For the energy equation we assumed parabolic profile of fluid velocity and the equality of temperatures and heat fluxes at the interface between the heated surface and the fluid. The void fraction was taken from a single-phase flow model. Two-dimensional temperature distributions were obtained by the Trefftz method and, due to the Robin condition at the interface between them, it was possible to calculate the heat transfer coefficient. Its values were compared to those obtained by other correlations known from literature.

Determination of the temperature distribution in a minichannel using ANSYS CFX and a procedure based on the Trefftz functions

This work discusses the mathematical model for laminar-flow heat transfer in a minichannel. The boundary conditions in the form of temperature distributions on the outer sides of the channel walls were determined from experimental data. The data were collected from the experimental stand the essential part of which is a vertical minichannel 1.7 mm deep, 16 mm wide and 180 mm long, asymmetrically heated by a Haynes-230 alloy plate. Infrared thermography allowed determining temperature changes on the outer side of the minichannel walls. The problem was analysed numerically through either ANSYS CFX software or special calculation procedures based on the Finite Element Method and Trefftz functions in the thermal boundary layer. The Trefftz functions were used to construct the basis functions. Solutions to the governing differential equations were approximated with a linear combination of Trefftz-type basis functions. Unknown coefficients of the linear combination were calculated by minimising the functional. The results of the comparative analysis were represented in a graphical form and discussed.

A numerical solution to an inverse unsteady-state heat transfer problem involving the Trefftz functions

This paper shows the results concerning flow boiling heat transfer in an asymmetrically heated vertical minichannel. The heated element for FC-72 Fluorinert flowing in that minichannel was a thin foil. The foil surface temperature was monitored continuously at 18 points by K-type thermocouples from the outer foil surface. Fluid temperature and pressure in the minichannel inlet and outlet, current supplied to the foil and voltage drop were also monitored. Measurements were carried out at 1 s intervals. The objective was to determine the heat transfer coefficient on the heated foil–fluid contact surface in the minichannel. It was obtained from the Robin boundary condition. The foil temperature was the result of solving the nonstationary two-dimensional inverse boundary problem in the heated foil. Using the FEM combined with Trefftz functions as basis functions solved the problem. The unknown temperature values at nodes were calculated by minimising the adequate functional. The values of local heat transfer coefficients were consistent with the results obtained by the authors in their previous studies when steady-state conditions were analysed. This time, however, these values were analysed as time dependent, which facilitated observation of coefficient changes that were impossible to observe under the steady-state conditions.