Boiling Pressure Drop Research Articles

Flow boiling pressure drop and heat transfer at high mass fluxes for R514a in a single microchannel

This paper presents pioneer experimental data about flow boiling heat transfer coefficient and pressure drop for R514a, which is a new environmentally friendly HFO refrigerant substitute for the HCFC R123. Data was collected in a single 1.1 mm internal diameter microchannel with high mass velocity conditions, ranging from 1500 to 7000 kg/m2s, allowing measurements of very high heat transfer coefficients, above 50 kW/m2K. In addition, saturation temperatures up to 92 °C were tested, which has an important application in evaporators of organic Rankine cycles. Comparisons were made between the experimental results of R514a and R123. R514a showed a general trend of a higher heat transfer coefficient and pressure drop (up to 20% higher) relative to R123. Frictional pressure drop literature correlations could predict the R514a data with a mean absolute error below 20% and heat transfer coefficient correlations with mean absolute error below 25%.

Pressure drop correlation for subcooled flow boiling in micro-channel heat sink

This study experimentally explores the subcooled flow boiling pressure drop of FC-72 in a multiple parallel rectangular micro-channel heat sink with a channel hydraulic diameter of 666 µm (width 400 µm and height 2000 µm). Detailed experimental procedures and data reduction methodology are presented. The study analyzes and reports the impact of inlet subcooling on the total pressure drop during subcooled flow boiling of FC-72 in rectangular micro-channels heat sink. Additionally, a consolidated subcooled flow boiling pressure drop database is created by incorporating the additional data points assimilated from the study of Lee and Mudawar (2008), which used HFE-7100 as the coolant fluid. The consolidated database is methodically reviewed and evaluated using the seminal pressure drop correlations described in the literature for pressure drop during subcooled flow boiling. For the micro-channels heat sink adopted in this study, existing subcooled flow boiling pressure drop correlations fail to predict the consolidated database. Based on the consolidated pressure drop data points, a new subcooled flow boiling pressure drop correlation for the rectangular micro-channels heat sink is suggested. The new pressure drop correlation well predicts the consolidated subcooled flow boiling pressure drop database with a mean absolute error of 16.6 % and 86.3 % of data points predicted inside ±30 %.

Flow boiling pressure drop correlation in small to micro passages

An accurate and acceptable correlation for the prediction of two-phase pressure drop is considered a crucial step in heat exchanger design. Although many existing models and correlations were developed and proposed in the past, their ability to predict within acceptable error bands when applied in general to flow boiling flows is limited, even within their originally recorded ranges. The discrepancies are worse when predicting two-phase pressure drop in small to micro-scale passages. Therefore, the aim of the work described in this paper is to assess the most well-known models and correlations using a large experimental data-bank. The data-bank includes four refrigerants, namely R134a, R245fa, HFE-7100 and HFE-7200, small to micro heat exchangers made of different metals and different channel configurations. In addition, the data cover a large range of operating conditions, which can allow generally applicability of new correlations developed. This range covers channel hydraulic diameter of 0.46−4.26 mm, heated length of 20−500 mm, system pressure of 1 − 14 bar, mass flux of 50−700 kg/m2 s, wall heat flux ranging from 2 to 234 kW/m2 and exit vapour quality up to one. Twenty six existing models and correlations that were developed and proposed for vertical/horizontal flow, single/multi-channels and circular/non-circular channels were evaluated. Moreover, the effect of using different equations for calculating the two-phase mixture viscosity, void fraction, Fanning friction factor and Lockhart–Martinelli parameter was assessed and discussed. The mean absolute error of the existing correlations when compared with our data-bank was more than 30 %. Therefore, a new correlation for calculating the two-phase multiplier, which was strongly dependent on the Boiling number and the Lockhart–Martinelli parameter, was developed and then the frictional component and total two-phase pressure drop relationships were completed.

An artificial neural network-based numerical estimation of the boiling pressure drop of different refrigerants flowing in smooth and micro-fin tubes

Abstract In thermal engineering implementations, heat exchangers need to have improved thermal capabilities and be smaller to save energy. Surface adjustments on tube heat exchanger walls may improve heat transfer using new manufacturing technologies. Since quantifying enhanced tube features is quite difficult due to the intricacy of fluid flow and heat transfer processes, numerical methods are preferred to create efficient heat exchangers. Recently, machine learning algorithms have been able to analyze flow and heat transfer in improved tubes. Machine learning methods may increase heat exchanger efficiency estimates using data. In this study, the boiling pressure drop of different refrigerants in smooth and micro-fin tubes is predicted using an artificial neural network-based machine learning approach. Two different numerical models are built based on the operating conditions, geometric specifications, and dimensionless numbers employed in the two-phase flows. A dataset including 812 data points representing the flow of R12, R125, R134a, R22, R32, R32/R134a, R407c, and R410a through smooth and micro-fin pipes is used to evaluate feed-forward and backward propagation multi-layer perceptron networks. The findings demonstrate that the neural networks have an average error margin of 10 percent when predicting the pressure drop of the refrigerant flow in both smooth and micro-fin tubes. The calculated R-values for the artificial neural network’s supplementary performance factors are found above 0.99 for all models. According to the results, margins of deviations of 0.3 percent and 0.05 percent are obtained for the tested tubes in Model 1, while deviations of 0.79 percent and 0.32 percent are found for them in Model 2.

Pressure drop for subcooled water boiling in microchannels under high heat fluxes: Experiments and predictions from artificial neural networks

Subcooled flow boiling in microchannels holds considerable promise in compact cooling applications. This study reports experimental measurements of the pressure drop of subcooled flow boiling of water in microchannels with inner diameters ranging from 0.5875 mm to 2 mm. The experiments are conducted over a range of operating parameters, including q = 3.0–10 MW/m2, p = 3.0–5.0 MPa, and G = 2000–10,000 kg/(m2∙s). The influences of diameter, heated length, and operating parameters are comprehensively analyzed. The results indicate that an increase in heat flux and pressure leads to an advancement of the pressure drop under the same thermodynamic vapor quality. Furthermore, a multi-layer artificial neural networks model optimized by a genetic algorithm is established using over 1000 experimental data. This model is employed to predict the subcooled boiling pressure drop and determine the optimal dimensionless parameter combination for developing a new pressure drop empirical correlation. The optimal combination of parameters includes Ja, ρl/ρg, Lsb/Lsat, Co and We, and the new correlation is proposed to calculate subcooled boiling pressure drop for high-heat-flux and microchannels applications, exhibiting a mean absolute error (MAE) of 10.64%. The results provide a novel approach that integrates machine learning techniques with empirical correlation, aiming to meet the requirements of the design optimization for thermal management solutions.

Flow boiling pressure drop of R-1270 in 1.0 mm tube

This study experimentally investigated the flow boiling pressure drop and flow patterns of R-1270 in a horizontal circular tube with 1.0 mm ID. The experiments were carried out with mass flux between 240 and 480 kg/(m2s) and heat flux ranging from 5 to 60 kW/m2 at a saturation temperature of 25 °C. Flow patterns and void fraction distribution among channels were observed with a high-speed camera in a visualization section. The influence of mass flux and vapour quality on pressure drop was verified. The flow patterns identified during tests were plug, slug churn and wavy-annular, differently distributed along the channels. The results were also compared with eight different correlations proposed in the literature. Results indicate the dependence of pressure drop on both mass and vapour quality.

Experimental study of flow boiling pressure drop and heat transfer of R1233zd(E) at moderate and high saturation temperatures

The paper presents the results of experimental study of flow boiling of R1233zd(E) in minichannel at moderate and high saturation temperatures. The heat transfer coefficient and pressure drop were measured for flow boiling in a stainless steel tube with 2 mm inside diameter and a length of 300 mm. Experiment has been conducted for saturation temperatures ranging from 83 to 145 °C which corresponds to values of reduced pressures ranging from 0.2 to 0.7. Three different heat fluxes were analyzed, namely 20, 30 and 45 kW/m2 and mass velocities ranged from 200 to 1000 kg/m2s. The effects of reduced pressure, mass velocity, heat flux, vapor quality and flow regime were analyzed. At low reduced pressures (pr≤ 0.3) heat transfer is driven by convective boiling mostly with nucleate boiling having effect only in the case of low mass velocities. Heat transfer in this case increases with mass velocity and vapor quality and is independent of heat flux. For reduced pressures of 0.4 and 0.5 both nucleate and convective boiling are present, however the nucleate boiling starts to be more dominant here, especially for low mass velocities and high heat fluxes. For low mass velocities and/or higher heat fluxes the heat transfer decrease or is independent of vapor quality, whereas for higher mass fluxes and/or lower heat fluxes it is initially independent of vapor quality and starts to increase close to the transition from intermittent to annular flow. At the range of high reduced pressures (pr ≥ 0.6) nucleate boiling is the dominant heat transfer mechanism. Experimental results were compared with some well-known two-phase flow correlations.

Flow boiling pressure drop and flow patterns of R-600a in a multiport minichannels

This study experimentally investigated the flow boiling pressure drop and flow patterns of R-600a in a horizontal multiport extruded tube with 1.47 mm ID. The experiments were carried out with mass flux between 35 and 165 kgm−2s−1 and heat flux ranging from 4.5 to 18.5 kWm−2 at a saturation temperature of 24 °C. Flow patterns and void fraction distribution amongst channels were observed with a high-speed camera in a visualization section. The influence of mass flux and vapour quality on pressure drop was verified. The flow patterns identified during tests were plug, slug churn and wavy-annular, differently distributed along the channels. The results were also compared with eight different correlations proposed in the literature. Results indicate the dependence of pressure drop on both mass and heat fluxes.

EXPERIMENTAL INVESTIGATION OF FLOW BOILING PARAMETERS IN A TRANSVERSE GROOVED HORIZONTAL TUBE: CRITICAL HEAT FLUX, PRESSURE DROP, AND HEAT TRANSFER COEFFICIENT

An experimental investigation was conducted to examine critical heat flux (CHF), boiling pressure drop, and local heat transfer coefficient in a horizontal channel with transverse grooves for flow boiling with R-123. Measurement of the wall temperature of the channel was done using a thermal camera. Five test sections were used for the study. A combination of groove pitch (<i>p</i>) of 18.6 and 11 mm and groove height (<i>e</i>) of 1 mm and 0.55 mm were used for the study. The groove pitch-to-height (<i>p/e</i>) ratios used were 11, 20, 18.6, and 33.8. Experiments were carried out for different heat fluxes in the range from 180 to 1259 kW/m<sup>2</sup>, six different mass fluxes from 180 to 1259 kg/m<sup>2</sup>s, and Reynolds numbers from 5834 to 51770. In the grooved channel compared with the smooth tube, an improvement in heat transfer and CHF with the higher pressure drop was found. The boiling heat transfer coefficient increases with a decrease in (<i>p/e</i>) ratio. The enhancement of heat transfer coefficient and CHF improves with a decrease in (<i>p/e</i>) ratio. The maximum enhancement of CHF for (<i>p/e</i>) ratios of 11, 18.6, 20, and 33.8 is found to be 134%, 87%, 57%, and 53%, respectively. An abrupt increase in test channel wall temperature is an indication of CHF occurring. CHF was observed at the top portion of the grooved channel and was found to be of departure from nucleate boiling type.

Flow boiling instability and pressure drop characteristics based on micro-pin-finned surfaces in a microchannel heat sink

In this work, flow boiling pressure drop characteristics based on the hydrodynamic instability with deionized water on the micro-pin-finned surfaces in a horizontal narrow microchannel with a hydraulic diameter of 952 µm were investigated. The micro-pin-finned surfaces can be divided into three types based on the size and arrangement of the fins (A30-120/S30-120/A50-120). The experiments were conducted under the mass flux from 200 to 500 kg/m2s and the inlet temperature of 30 to 50 ℃. The two-phase flow patterns on the micro-pin-finned surfaces were captured by a high-speed camera. The flow boiling instabilities in the microchannel are studied, and the two-phase flow pattern, wall temperature, heat fluxes and mass fluxes as well as pressure drop are observed and analyzed. The experiment results show the mass flux has a significant effect on the pressure drop, while it has almost no change with the increasing heat flux in the microchannel heat sink during the nucleate boiling. Besides, the pressure drop of the A30-120 and S30-120 surfaces is less than that of smooth surface due to the effect of hydrophobic surface and bubble drag reduction. The maximum pressure drop of the A30-120 surface is only 0.970 kPa corresponding to the mass flux and critical heat flux (CHF) of 500 kg/m2s and 341.4 W/cm2 in this study, respectively. And the overall performance of the micro-pin-finned surfaces is also evaluated in the microchannel heat sink, which can achieve more than a 30% reduction in pressure drop and thermal resistance compared with the smooth surface.

New flow boiling frictional pressure drop multipliers for smooth and microfin tubes

Abstract The accurate calculation of pressure drop of evaporators/condensers are crucial and related with the pumping power, performance coefficient and energy consumption in a refrigeration equipment. This work aligns frictional pressure drop models/correlations with the experimental outcomes of boiling pressure drop of R134a in horizontal smooth and microfin copper tubes with equivalent outer diameter of 9.52 mm. The pressure drop through the test tube is obtained with a differential pressure transducer directly. Effective parameters are specified for smooth and microfin tubes and the most compatible models/correlations, 12 for smooth tubes and 9 for microfin ones, are determined accurately in relation to the consequences of investigation during intermittent and annular flow regime. Moreover, new two-phase multipliers have been developed by using regression analyses of 182 data points based on Lockhart-Martinelli parameter for each test tubes separately, and their predictability are found to be better than others in the literature as novel ones. Average errors of the developed empirical correlations are 11% for smooth and for 7% for microfin tubes. Finally, the measured data is given for the validation issues of researchers who can benefit from most of the investigated pressure drop models with tolerable accuracy regarding with their HEX design analyses.

Adiabatic frictional pressure gradient during flow boiling of pure refrigerant R1233zd and non-azeotropic mixtures R448A, R452A and R455A

The research on two-phase flow characteristics of refrigerants is of primary importance in several fields, such as air conditioning and refrigeration systems. Therefore, the determination of the pressure drop during flow boiling is important for the correct design of evaporators and heat spreaders systems. This paper presents a collection of experiments on flow boiling pressure drop using pure refrigerant R1233zd and new low-GWP refrigerant mixtures R448A, R452A and R455A. All tests were performed in adiabatic conditions, in a smooth horizontal stainless-steel tube having an internal diameter of 6.0 mm and a thickness of 1.0 mm. The effect of operating parameters, such as (bubble) saturation temperature (from 25 to 65 °C) and mass flux (from 150 to 600 kg/m2s) is investigated and discussed, and the performance of the chosen fluids is also compared. Finally, an assessment of existing prediction methods is carried-out to find the most suitable correlations for two-phase pressure drop evaluation.

An experimental study on the subcooled flow boiling pressure drop of R141b in the system of two microchannels

An experimental study of the pressure drop under subcooled flow boiling of the refrigerant R141b in a system with two slotted microchannels was carried out. A copper block with two microchannels 2 mm wide, 0.4 mm deep, and 16 mm long was used as an experimental section for testing. The mass flow rate varied in the range from 1 to 4 g/s, the initial subcooling from 20°C to 50°C. Experimental data show a significant decrease in the pressure drop when the critical heat flux is reached. The experimental data are compared with the model known from the literature. Experimental data show that the occurrence of nucleate boiling incipience at subcooled boiling corresponds to a larger heat flux than that given by the recommended correlation.

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%.

Machine learning algorithms to predict flow boiling pressure drop in mini/micro-channels based on universal consolidated data

Two-phase flow in mini/micro-channels can meet the high heat dissipation requirements of many state-of-the-art cooling solutions. However, there is lack of accurate universal methods for predicting parameters like pressure drop in these configurations. Conventional ways of predicting pressure drop employ either Homogeneous Equilibrium Model (HEM) or semi-empirical correlations. This current study leverages the availability of data collected over the past few decades to build several machine learning models to demonstrate the efficacy and ease of building and deploying such models. A consolidated database of 2787 data points for flow boiling pressure drop in mini/micro-channels is amassed from 21 sources that includes 10 working fluid, reduced pressures of 0.0006 –0.7766, hydraulic diameters of 0.15–5.35 mm, mass velocities of 33.1 < G < 2738 kg/m 2 s, liquid-only Reynolds numbers of 14–27,658, superficial vapor Reynolds number of 75.58–199,453 and flow qualities of 0 and 1. This consolidated database is utilized to develop four machine learning based regression models viz., Artificial Neural Networks (ANN), KNN regression, Extreme Gradient Boosting (XGBoost) and Light GBM. Both input parameters and hyperparameters are optimized for the individual models. The models with dimensionless input parameters: Bd, Bo, Frf, Frfo, Frg, Frgo, Frtp, Prf, Prg, Peg, Pef, Ref, Refo, Reg, Rego, Reeq, Suf, Sug, Wef, Wefo, Weg, Wego,Wetp predict the test data for ANN model, XGBoost model, KNN model, and LightGBM model with MAEs of 9.58%, 10.38%, 13.52%, and 14.49%, respectively. The optimized machine-learning models performed better than highly reliable generalized pressure drop correlations plus showed good performance across individual datasets, flow regimes, and channel configurations.

Study on the subcooled boiling pressure drop for downward flow in a narrow rectangular channel

Subcooled boiling pressure drop is an issue worthy of attention for heat exchange systems, such as nuclear reactor core. Few related studies focused on the downward flow in narrow rectangular channel. In this study, the subcooled boiling pressure drop is obtained from an experiment in a narrow rectangular channel (2.3 mm×67 mm) under downward flow with low heat flux, mass flux and pressure. To calculate the pressure drop multiplier Φ2, a correlation of single-phase adiabatic friction factor suitable for the present experiment is proposed and a new method is used to deal with the Δpsc. The experimental data are compared with previous empirical correlations of subcooled boiling pressure drop, and their predictive abilities for this experiment data are evaluated respectively. An empirical correlation for predicting subcooled boiling pressure drop is developed considering the effect of heat and mass flux, inlet subcooling and pressure. It shows good performance with the experimental data.

Flow boiling pressure drop characteristics in a multi-microchannel heat sink

We make an effort in this study to experimentally investigate the flow boiling pressure drop characteristics in a multi-microchannel heat sink. For the multi-microchannel heat sink, 27 parallel microfluidic channels are considered. Microchannels are fabricated on the copper block and have a hydraulic diameter of 421 µm and length of 40 mm. We perform experiments considering the refrigerant, considered the working fluid in this study. A high mass flux (G ∼ 400 kg/m2 s–1200 kg/m2 s) for a range of wall heat flux (q′′ ∼ 10 kW/m2–170 kW/m2) and varying degrees of saturation temperature (Tsat ∼ 13 °C, 18 °C, and 23 °C) are considered for the experiments. We demonstrate the correlative effect of the total pressure drop on the two-phase flow pattern in the channel. By depicting the interplay between the total pressure drop (∆p) and the frictional pressure gradient, we show that the frictional pressure gradient increases with the increasing mass flux while it decreases with the increasing degree of saturation temperature. We show the significant impact of the heat flux and saturation temperature on the frictional pressure gradient. In addition, we develop a new correlation for the pressure drop characteristics taking the interference effect of the frictional pressure drop in the multi-microchannel heat sink. The inferences of this experimental study will have far-ranging consequences for the design of heat exchangers, leading to the optimization of microscale thermal management equipment.

Effect of aspect ratio on flow boiling characteristics in microchannels

The effect of channel aspect ratio on flow boiling characteristics (flow patterns, heat transfer and pressure drop) of HFE-7100 in copper multi-microchannel heat sinks was investigated experimentally. Three heat sinks with base area 500 mm2, channel hydraulic diameter 0.46 mm and channel aspect ratio (ratio of channel width to channel height) of 0.5, 1.0 and 2.0 were tested. The average surface roughness of the channel bottom surface was nearly the same in the three heat sinks and the measured values were 0.271, 0.286 and 0.304 µm. The local heat transfer rates were determined simultaneously with flow visualisation at mass flux ranging from 50 to 250 kg/m2s, wall heat flux from 9.6 to 191.6 kW/m2, system pressure of 1 bar and low inlet sub-cooling of 5 K. The results showed that, when the channel aspect ratio increased, the heat transfer coefficient increased, while the flow boiling pressure drop decreased. However, the heat transfer rate calculated using the heat sink base area was higher in the heat sink with the smallest channel aspect ratio, indicating an enhancement due to the largest surface area.

Pressure drop, local heat transfer coefficient, and critical heat flux of DNB type for flow boiling in a horizontal straight tube with R-123

As far as design safety and operation of heat exchangers with a horizontal flow are concerned, it is necessary to study local heat transfer coefficient, boiling pressure drop, and critical heat flux for flow in a horizontal tube. In the present experimental work, local axial distribution of heat transfer coefficient, two-phase pressure drop, and critical heat flux for the flow boiling in a horizontal straight tube with R-123 as working fluid has been studied. Experiments are performed in horizontal tubes of diameter 11.9 mm and wall thickness 0.4 mm of SS304, having a heated length of 400 mm, 600 mm, and 1000 mm for the mass flux of 180 to 1210 kg/m2s. The local wall temperature is measured using the Infra-Red thermal imaging technique. The local heat transfer coefficients are compared with six different well-known correlations. Also, the two-phase frictional pressure drop is measured and compared with eight different general correlations. In the present study, a sudden rise in wall temperature at any location of a test section is considered as the occurrence of a boiling crisis. The boiling crisis mechanism observed in the present study is of departure from nucleate boiling (DNB). It takes place in the subcooled, or low quality saturated boiling region and in-between the length of the test sections rather than at exit as in the dry-out type of CHF. The critical heat flux is compared with six different predictive correlations. The mechanism of occurrence of CHF in the present work is found to be a departure from nucleate boiling.

Comparison of flow boiling pressure drop and heat transfer of R134a with low GWP alternative R1234ze(E) in a dimpled flat duct

Abstract The low GWP (Global Warming Potential) refrigerant R1234ze(E) is a potential alternative of R134a. This study puts the emphasis on the comparison of the flow boiling pressure drop and heat transfer of R134a and R1234ze(E) in a dimpled flat duct. For the air-conditioning applications, the experiments are conducted at mass flux from 100 to 200 kg m−2 s−1, saturation temperature from 5 to 15 °C, heat flux from 2.5 to 10 kW m−2, and vapor quality from 0.1 to 0.95. The results show that the frictional pressure gradient of R1234ze(E) is 1.10 to 1.27 times that of R134a. The characteristics of the flow boiling heat transfer coefficient of R1234ze(E) are similar to R134a. The increasing flow boiling heat transfer coefficient with vapor quality in the tested range implies the convective evaporation dominates the heat transfer. It is also observed that the flow boiling heat transfer coefficient visibly increases with a rise in mass flux and decreases with the saturation temperature increment due to the increase in vapor to liquid density ratio. The enhancement effect of heat flux on the flow boiling heat transfer shows up at the highest value of 10 kW m−2 only. The heat transfer coefficient ratio of R1234ze(E) to R134a increases from 0.89 to 1.20 with an increase in vapor quality. The comparison of frictional pressure gradient and heat transfer characteristics between R1234ze(E) and R134a has been discussed from the viewpoint of fluid properties.