Vertical Minichannel Research Articles

Flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel

This article reports on flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel. The experiments were carried out with a resistively heated stainless steel tube (1.60mm in diameter and 245mm heated length) at 27 and 32°C saturation temperature. Five mass fluxes in the range 100–500kg/m2s with heat fluxes from 5 to 245kW/m2 were tested. Under similar operating conditions experiments were repeated with R134a in the same setup to compare thermal performance of R152a. The results showed that the heat transfer was strongly influenced by the applied heat flux with insignificant convective contributions. The dryout heat flux increased with increasing mass flux but no effect of varying operating pressure was noticed. The experimental results for heat transfer and dryout heat flux were compared with well-known macro and micro-scale correlations from the literature.

Heat transfer mechanism, pressure drop and flow patterns during FC-72 flow boiling in horizontal and vertical minichannels with enhanced walls

The paper discusses the results of the flow boiling heat transfer in horizontal (in two positions) and vertical minichannels of 1mm depth, 40mm width and 360mm length. A single-sided enhanced foil with various depressions heats FC-72 laminar flow. Liquid crystal thermography was used for measuring the temperature distribution on the plain side of the foil. The observations of the flow structures were carried out on the enhanced side of the foil contacting fluid in the minichannel, which enabled the identification of flow structures and presentation of flow pattern maps. The void fraction was determined for some cross-sections of two-phase flow images and the vapour quality was determined on this basis. The study employed heat transfer analysis and results regarding local heat transfer coefficient, and boiling curves were constructed. In addition to the typical shapes of boiling curves, the untypical boiling curves with several “stepped” courses of nucleation hysteresis were found. The suitability of classical methods of the two phase pressure drop determination using the experimental verification was confirmed. The comparison of the total measured pressure drop with data obtained from the theoretical models shows that all predicted results fall within the range of ±30%, but only for the vertical minichannel. In most cases the experimental data reached values approaching approximate values in comparison with values from the theoretical models. Calculation of the pressure drop on the basis of the thermodynamic vapour quality and the vapour quality calculated from the experimentally determined void fraction showed a similar dependence.

An experimental investigation of flow boiling heat transfer of R-134a in horizontal and vertical mini-channels

Flow boiling experiments with R-134a flowing in horizontal and vertical mini-channels are performed to obtain flow visualization and heat transfer data. A 1.75mm diameter channel with a length of 600mm is used as test section. Two sets of the horizontal flow and vertical upward flow data are presented based on a mass flux range of 200–1000kg/m2s, a heat flux range of 1–80kW/m2 and a saturation pressure range of 7–13bar. Flow visualization and heat transfer results show heat transfer coefficient depending on flow pattern. The results also indicate that flow regime transition and heat transfer coefficient tend to depend on flow direction under the certain experimental conditions. In addition, the comparisons between the experimental data and the predictions recommended for either macro-scale flow boiling or micro-scale flow boiling are presented in this work.

615 垂直細管内の環状二相流における液膜厚さと液滴流量率(流体力学4)

615 垂直細管内の環状二相流における液膜厚さと液滴流量率(流体力学4)

An application of enhanced heating surface with mini-reentrant cavities for flow boiling research in minichannels

The paper presents the results of flow boiling heat transfer in a minichannel 1.0 mm deep and 40 mm wide, vertically oriented. The heating element for the working fluid (FC-72) that flows along the vertical minichannel is thin, single-sided enhanced alloy foil. In the selected area or on the entire enhanced side of the heating foil mini-reentrant cavities distributed unevenly were formed. Liquid crystal thermography has been used for measuring the two-dimensional temperature distribution on the plain side of the heating foil. Simultaneously, the observations of the flow structures have been carried out on the enhanced side of the heating foil contacting fluid in a minichannel. The results are presented as void fraction dependence along the minichannel length for the selected cross-sections. Exemplary boiling curves obtained from initial increasing and subsequent decreasing the heat flux supplied to the foil are also presented.

Comparison of experimental data and numerical simulation of two-phase flow pattern in vertical minichannel

Comparison of experimental data and numerical simulation of two-phase flow pattern in vertical minichannel The aim of the study was the implementation of a numerical simulation of the air-water two-phase flow in the minichannel and comparing results obtained with the values obtained experimentally. To perform the numerical simulations commercial software ANSYS FLUENT 12 was used. The first step of the study was to reproduce the actual research installation as a three-dimensional model with appropriate and possible simplifications - future computational domain. The next step was discretisation of the computational domain and determination of the types of boundary conditions. ANSYS FLUENT 12 has three built-in basic models with which a two-phase flow can be described. However, in this work Volume-of-Fluid (VOF) model was selected as it meets the established requirements of research. Preliminary calculations were performed for a simplified geometry. The calculations were later verified whether or not the simplifications of geometry were chosen correctly and if they affected the calculation. The next stage was validation of the chosen model. After positive verification, a series of calculations was performed, in which the boundary conditions were the same as the starting conditions in laboratory experiments. A satisfactory description of the experimental data accuracy was attained.

Flow boiling of ammonia in vertical small diameter tubes: Two phase frictional pressure drop results and assessment of prediction methods

Two phase pressure drops were measured for ammonia at a wide range of test conditions in two sizes of vertical mini channels. The test sections were made of stainless steel (AISI 316) tubes with internal diameter of 1.70 mm and 1.224 mm and a uniformly heated length of 245 mm. Experiments were conducted at three saturation temperatures of 23 °C, 33 °C and 43 °C, the heat flux ranging from 15 to 355 kW/m2 and the mass flux ranging from 100 to 500 kg/m2s. The effect of the heat flux, the mass flux, the vapour quality, the saturation temperature and the internal diameter on the two phase pressure drop are presented in this article. Some generalized two phase pressure drop correlations suggested for macro and micro scale channels are examined by comparing them with our experimental data. None of the examined correlations agreed well with the test data. A new correlation (modified form of Tran et al. correlation) is proposed which is able to predict the experimental data with MAD of 16% and 86% of the data is within ±30% range.

Gas and Liquid Two-Phase Flow Injected Directly into Vertical Mini-Channel

Gas and Liquid Two-Phase Flow Injected Directly into Vertical Mini-Channel

The solution of the two-dimensional inverse heat transfer problem with the use of the FEM in combination with Trefftz functions

2 Abstract: The aim of this paper is to determine the boiling heat transfer coefficient for the cooling liquid flow in a rectangular minichannel with asymmetric heating. The main part of the test section is made up of a vertical minichannel of 1.0 mm depth. The heating foil on the side of the fluid flowing in the minichannel is single- sided enhanced on the selected area. The experiment is carried out with FC-72. The investigations focus on the transition from single-phase forced convection to nucleate boiling, that is, from the zone of boiling incipience further to developed boiling. Owing to the liquid crystal layer located on the heating surface contacting the glass, it is possible to measure the heating wall temperature distribution while increasing the heat flux transferred to the liquid flowing in the minichannel. The objective of the calculations is to evaluate a heat transfer model and numerical approach to solving the inverse boundary problem, and to calculate the heat transfer coefficient. This problem has been solved by means the finite element method in combination with Trefftz functions (FEMT). Trefftz functions are used to construct base functions in Hermite space of the finite element.

Boiling heat transfer of ammonia in vertical smooth mini channels: Experimental results and predictions

In this article, flow boiling heat transfer results of ammonia in mini channels are reported. Experiments have been performed to investigate heat transfer in circular vertical mini channels made of stainless steel (AISI 316) with internal diameters of 1.70 mm and 1.224 mm and a uniformly heated length of 245 mm. The test conditions are: mass flux ranging from 100 to 500 kg/m2s, heat flux ranging from 15 to 355 kW/m2 and saturation temperatures of 23 °C, 33 °C and 43 °C. The effects of mass flux, heat flux, vapour quality, saturation temperature and internal diameter on heat transfer coefficients are explored in detail. The local heat transfer coefficients of ammonia with 1.70 mm tube at all vapour qualities and at lower vapour qualities with 1.224 mm tube, are more or less independent of mass flux and vapour quality and are a function of heat flux while the local heat transfer coefficients with 1.224 mm tube at higher vapour qualities are function of mass flux and vapour quality and independent of heat flux. The heat transfer coefficient is observed to be higher for lower internal diameter tube. The heat transfer coefficient is higher for higher saturation temperature at lower vapour qualities and no effect of saturation temperature is observed at higher vapour qualities for both test sections. The experimental data is compared with well known correlations and among them, the Cooper’s [1] correlation gave best predictions if all data points are included.

Experimental investigation of dryout of propane in uniformly heated single vertical mini-channels

This article presents dryout results of propane in single vertical circular minichannels made of stainless steel with internal diameters of 1.70 mm and 1.224 mm. The heat flux is increased in steps up to occurrence of dryout. The effects of different parameters such as mass flux, saturation temperature, vapour quality and internal diameter on the dryout are investigated. The results show that the dryout heat flux increases with the increase in mass velocity, with the decrease of vapour quality and with the increase of internal diameter. Almost no effect of saturation temperature on dryout heat flux is observed. Generalised CHF correlations developed for macro and micro scale from the literature are also compared with the experimental results.

F24 垂直ミニチャンネル内気液二相流における摩擦圧力勾配の予測(F2気液二相流)

F24 垂直ミニチャンネル内気液二相流における摩擦圧力勾配の予測(F2気液二相流)

Experimental study on condensation heat transfer in vertical minichannels for new refrigerant R1234ze(E) versus R134a and R236fa

Experimental condensation heat transfer data for the new refrigerant R1234ze(E), trans-1,3,3,3-tetrafluoropropene, are presented and compared with refrigerants R134a and R236fa for a vertically aligned, aluminum multi-port tube. Local condensation heat transfer measurements with such a multi-microchannel test section are very challenging due to the large uncertainties related to the heat flux estimation. Presently, a new experimental test facility was designed with a test section to directly measure the wall temperature along a vertically aligned aluminum multi-port tube with rectangular channels of 1.45 mm hydraulic diameter. Then, a new data reduction process was developed to compute the local condensation heat transfer coefficients accounting for the non-uniform distribution of the local heat flux along the channels. The condensation heat transfer coefficients showed the expected decrease as the vapor quality decreased (1.0–0.0) during the condensation process, as the mass velocity decreased (260–50 kg m −2 s −1) and as the saturation temperature increased (25–70 °C). However, the heat transfer coefficients were not affected by the condensing heat flux (1–62 kW m −2) or by the entrance conditions within the tested range. It was found that the heat transfer performance of R1234ze(E) was about 15–25% lower than for R134a but relatively similar to R236fa. The experimental data were then compared with leading prediction methods from the literature for horizontal channels. In general, the agreement was poor, over-predicting the high Nusselt number data and under-predicting the low Nusselt number data, but capturing the mid-range quite well. A modified correlation was developed and yielded a good agreement with the current database for all three fluids over a wide range of operating conditions.

Statistic characteristics of the gas-liquid flow in a vertical minichannel

The gas-liquid upward flow was studied in a rectangular minichannel of 1.75×3.8 mm and length of 0.7 m. The experiments were carried out within the range of the gas superficial velocity from 0.1 to 10 m/s and the liquid superficial velocity from 0.07 to 0.7 m/s for the co-current H2O/CO2 flow under the conditions of saturation. The method for the two-beam laser scanning of structure and determination of statistic characteristics of the two-phase flow was worked through. The slug-bubble, slug, transitional, churn, and annular flows were distinguished. The statistics characteristics of liquid and gas phases motion in a minichannel were obtained for the first time including the velocities of phase motion.

Velocity and gas-void fraction in two-phase liquid-gas flow in narrow mini-channels

In the study, the values of velocity and gas void fraction in two-phase gas-liquid flow in narrow vertical mini-channels were experimentally determined. The influence of the physicochemical properties of liquid, the channel gap width and the superficial velocity of gas and liquid, on gas void fraction and gas bubble flow velocities in the flowing two-phase mixture in narrow, vertical mini-channels was investigated. The value of gas phase velocity was defined by means of a drift model, determining the values of distribution parameter C 0 . The relation describing the value of gas void fraction depended on the Bankoff coefficient and gas input volume fraction.

BOILING HEAT TRANSFER IN A VERTICAL MICROCHANNEL: LOCAL ESTIMATION DURING FLOW BOILING WITH A NON INTRUSIVE METHOD

This paper summarizes the results of experimental and numerical studies concerning boiling heat transfer inside vertical minichannels. The objective here is to provide basic knowledge on the systems of biphasic cooling in minichannels for several gravity levels (μg, 1g, 2g). To fully understand the high heat transfer potential of boiling flows in microscale’s geometry, it is vital to quantify these transfers. To achieve this goal, an experimental device has been set up and in order to study the influence of gravity, the device has been embarked on board A300 Zero-G to make Parabolic Flights experiments. Analysis is made up by using an inverse method in order to estimate the local heat coefficient while boiling occurs inside a minichannel. The results concern two-phase flow during convective boiling and this investigation leads to solve an inverse heat conduction problem (IHCP). The estimation consists in inversing experimental data measurements located in the heating rod of our experiment to obtain the wall temperature and the heat flux on the minichannel surface. Images and video sequences have been performed with a high speed camera. The experiments are conducted with HFE-7100 as this fluid has a low boiling temperature at the cabin pressure of the A300. This choice was governed by the capillary length which is 4.3 mm in microgravity (0.05 m.s). The experimental loop allows the control of the heat flux and the liquid flow rate for three hydraulic diameters (DH): 0.49 mm; 0.84 mm and 1.18 mm. The influence of three different parameters on the heat transfer coefficient are carried out: the gravity level, the Reynolds number and the vapor quality. First results show that whatever the gravity level, the local heat transfer decreases sharply from the inlet to the outlet channel. In the annular slug regime, the average heat transfer is found around 6000 W.m.K and we observe that the heat transfer coefficient is higher in microgravity (30 %) compared to normal and hypergravity conditions.

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

The paper presents the results of experimental and numerical studies of boiling heat transfer in the flow of refrigerants R123 and R11 through vertical, rectangular minichannels, with one wall heated. An application of liquid crystal thermography has helped detect two-dimensional temperature distribution on the heating surface, allowing determination of boiling heat fluxes and experimental boiling curves. The main objectives of the paper included the development of two-dimensional approach to solve the inverse heat conduction boundary problem for determining local values of internal heating surface temperature, boiling heat flux and heat transfer coefficient, and the improvement of the applied numerical method making use of the equalizing calculus and heating surface temperature measurement errors. A detailed discussion of temperature, heat flux and heat transfer coefficient errors is also provided.

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m2s and G = 50 kg/m2s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer.

Friction losses and heat transfer of single-phase flow in a mini-channel

Experimental frictional pressure drop and heat transfer during single phase flow in a vertical mini-channel have been studied with the aim of determining the validity of classical correlations available for conventional size channels. A 1 mm square channel etched in a 420 mm long test section of aluminum has been investigated. The Reynolds number has been varied from 310 to 7780 in order to cover the laminar regime as well as the beginning of the turbulent regime. The heat flux supplied to the fluid varies from 1 kW/m 2 to 8 kW/m 2. Experimental frictional pressure drop measurements show that classical correlations accurately apply. Temperature measurements along the channel show that the temperature profile is drastically different from the expected linear behaviour owing to an important longitudinal heat flux in the channel wall. This heat flux mal-distribution which has been recently discussed in the literature is clearly shown and studied in more details by a numerical simulation of the experiment. This numerical work has allowed to make a correction on temperature measurements. Once corrected, the heat transfer measurements are in fair agreement with the classical literature results.

Simulations numériques d’écoulements diphasiques gaz-liquide par la méthode de volume fini dans des géométries confines

In this paper we present results of volume-of-fluid method based direct numerical simulations of two types of gas-liquid flow in confined geometries. The first flow under consideration is the co-current upward flow of a regular train of gas bubbles within silicon oil in a square vertical mini-channel. Particular attention is given to the influence of the length of the flow unit cell and on the relation between bubble diameter and capillary number. The second type of flow under investigation is that of a swarm of eight bubbles within a narrow plane vertical channel. The numerical data are used to evaluate the budget of the turbulence kinetic energy equation for the liquid phase and to test common closure assumptions for the interfacial term appearing in this equation.