Mass Flux Range Research Articles

Experimental research and numerical simulation on cryogenic line chill-down process

The empirical heat transfer correlations are suggested for the fast cool down process of the cryogenic transfer line from room temperature to cryogenic temperature. The correlations include the heat transfer coefficient (HTC) correlations for single-phase gas convection and film boiling regimes, minimum heat flux (MHF) temperature, critical heat flux (CHF) temperature and CHF. The correlations are obtained from the experimental measurements. The experiments are conducted on a 12.7 mm outer diameter (OD), 1.25 mm wall thickness and 7 m long stainless steel horizontal pipe with liquid nitrogen (LN2). The effect of the lengthwise position is verified by measuring the temperature profiles in near the inlet and the outlet of the transfer line. The newly suggested heat transfer correlations are applied to the one-dimensional homogeneous transient model to simulate the cryogenic line chill-down process, and the chill-down time and the cryogen consumption are well predicted in the mass flux range from 26.0 kg/m2 s to 73.6 kg/m2 s through the correlations.

Two-dimensional heat transfer coefficients with simultaneous flow visualisations during two-phase flow boiling in a PDMS microchannel

Infrared (IR) thermography was combined with simultaneous high speed flow visualisation and pressure measurements from integrated pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient (HTC) maps across the full domain of a polydimethylsiloxane (PDMS) high aspect ratio (a = 22) microchannel (Dh = 192 μm). High spatial and temporal resolution two-dimensional wall temperature measurements and pressure data were obtained for a range of mass fluxes (G = 7.37–298 kg m−2 s−1) and heat fluxes (q = 13.64–179.2 kW m−2) using FC-72 as the working liquid. The 3D plots of HTC provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow regime. The optical images from the channel top revealed the local flow regimes and were correlated with simultaneous thermal images obtained from the channel base. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with vapour-liquid dynamics and liquid film thinning (from the contrast of the optical images) which caused suspected dryout. The correlation between the synchronised at the same frame rate high-resolution thermal and optical images will assist in a better understanding of the heat transfer mechanisms during two-phase flow boiling in microchannels. This work intends to give a better insight into heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during a cycle of liquid–vapour alternations.

Prediction of frictional pressure drop of R134a during condensation inside smooth and corrugated tubes

In the present paper, in order to understand the accuracy of 38 different correlations derived by various researchers in this field, the correlations were executed for condensation frictional pressure drop. To accomplish this goal, experimental data provided from authors' previous publications encompassing 412 points for two smooth tubes, and 334 points for five corrugated tubes, have been utilized so as to compare the determined results. The experimental setup is composed of a 2.5m double tube for horizontal configuration and smooth and corrugated tubes at the inner diameters of 0.0081m, while the applied mass flux range spans between 709 and 1974kgm−2s−1. The average quality of vapor and saturation pressure ranges lie within 0.09 to 0.97, and 10 to 13bar, respectively. Determining the frictional pressure drop in two-phase flows does not involve corrugated tube geometry in the calculation of friction factor, to make this available, a slight alteration that requires the replacement of a correlation with another one in the literature was suggested with regard to friction factor approach. As a result of this, it was noticed that performances of some correlations were optimized to predict the frictional pressure drop in corrugated tubes. Additionally, the most effective correlations have been selected for the horizontal double pipe heat exchanger having smooth and corrugated tubes. Finally, alteration of the condensation pressure drop with Reynolds number are presented using both experimental data and best predictive correlations.

Condensation flow patterns and heat transfer in horizontal microchannels

An experimental investigation was carried out to study the effect of refrigerant mass flux, local vapour quality, coolant flow rate and inlet coolant temperature on the local condensation heat transfer coefficient. Flow visualization was also conducted to capture flow patterns during flow condensation using a high-speed camera integrated with a microscope. HFE-7100, a dielectric and eco-friendly refrigerant was used in rectangular multi-microchannels with a hydraulic diameter of 0.57mm. Experiments were performed at a saturation temperature of 60°C, mass flux range 48–126kg/(m2s), coolant flow rate range 0.5–1.1L/min and inlet coolant temperature range 20–40°C. The results showed that the local condensation heat transfer coefficient increases with increasing mass flux and decreases with decreasing local vapour quality. A negligible effect of the coolant side conditions, saturation-to-wall temperature difference, on the local condensation heat transfer coefficient was found. The main flow regime was annular flow, while slug and bubbly flow were found at some operating conditions. The experimental results were compared with the existing correlations for heat transfer rates. Also, two existing flow pattern maps, for conventional and mini/microchannels, were used to compare the current flow pattern results.

Experimental Investigation of Condensation Heat Transfer and Adiabatic Pressure Drop Characteristics Inside a Microfin and Smooth Tube

Experiments on condensation heat transfer and adiabatic pressure drop characteristics of R134a were performed inside smooth and microfin horizontal tubes. The tests were conducted in the mass flux range of 50[Formula: see text]kg/m2s to 200[Formula: see text]kg/m2s, vapor quality range of 0 to 1 and saturation temperature range of 20[Formula: see text]C to 35[Formula: see text]C. The effects of mass velocity, vapor quality, saturation temperature, and microfin on the condensation heat transfer and frictional pressure drop were analyzed. It was discovered that the local heat transfer coefficients and frictional pressure drop increases with increasing mass flux and vapor quality and decreasing with increasing saturation temperature. Higher heat transfer coefficient and frictional pressure drop in microfin tube were observed. The present experimental data were compared with the existing well-known condensation heat transfer and frictional pressure drop models available in the open literature. The condensation heat transfer coefficient and frictional pressure drop of R134a in horizontal microfin tube was predicted within an acceptable range by the existing correlation.

Condensation heat transfer characteristics and flow regime in a horizontal rectangle channel of a multichannel cylinder dryer

ABSTRACTThe present study investigates the phenomenon of steam condensation in a rectangle horizontal channel with a mass flux range of 20–70 kg/m2/s and a vapor quality of 0.2–0.8. Mass flux and vapor quality were presented to primarily affect the heat transfer coefficient in shear-dominated flow regime, such as the mass flux is greater than 40 kg/m2/s. At the low mass flux, only slug flow, plug flow, and wave flow were observed. At the high mass flux, the wave flow and annular flow were observed even at the low-vapor quality, and the slug and plug flow were not found during the entire condensation process. Observed flow patterns agreed with Tandon’s flow pattern map. Four kinds of correlations are employed to predict the condensing heat transfer coefficient, and the result reveals that Cavallini’s correlation agrees with the experimental results relatively well. The measured pressure drop has a good agreement with the Lockhart–Martinelli correlation.

Experimental investigation on flow boiling characteristics of zeotropic binary mixtures (R134a/R245fa) in a rectangular micro-channel

In the present study, the flow boiling characteristics of R134a/R245fa zeotropic mixtures with three mass fractions (10/90, 30/70 and 70/30 by wt%) in a single rectangular micro-channel and their pure components were experimentally investigated. The flow boiling in micro-channel was visualized from the top view and heated on three sides with the cross-sectional area of 1mm×1mm and length of 106mm. For each test refrigerant, the visualization results and heat transfer coefficients were obtained under the parametric conditions comprising the heat flux range of 30–120kW/m2 and mass flux range of 60–1100kg/m2s at the same evaporating temperature of 18.5°C. The corresponded outlet vapor quality was in the range of 0–1. Results showed that almost all of the test refrigerants experienced five typical flow patterns. Different bubble coexistence phenomena were observed for the pure and mixed refrigerants, respectively. The hysteresis of flow pattern transition for R134a/R245fa zeotropic mixtures was strongly affected by both of the temperature glide and blending ratio. Based on the comparison with pure components, flow boiling heat transfer characteristics of test zeotropic mixtures were analyzed and discussed in detail by considering the mixture effects. Taking account of capillary and Marangoni effect in confined space, as well as other impact factors, the developed prediction method exhibited satisfactory accuracy in predicting the heat transfer coefficient of zeotropic mixtures.

Validation and model sensitivity of the interfacial area transport equation in condensing flows

The interfacial area transport equation (IATE) in condensing flows is thoroughly reviewed and validated with available experimental data. The IATE provides a dynamic prediction of flow structure consistent with the two-fluid model and has been studied extensively for adiabatic two-phase flows. However, the IATE still requires attention in flows with phase change. The modeling of the condensation interfacial area term and volume sink term is reviewed along with the void fraction prediction since the IATE is closely related to the void fraction development. The emphasis in this work is to expand the validation of the IATE in condensing flows to higher pressure and higher flow rate conditions as well as evaluating the modeling sensitivity in both the decoupled IATE and the IATE coupled with the void transport equation. Two datasets have been used for validation, both consisting of condensing two-phase flow data in the unheated section of a vertical annulus with conditions spanning a range of pressure, inlet subcooling and mass flux. The analysis shows that the current form of the condensation modeling results in an over-prediction of both condensation mass transfer and interfacial area concentration loss across all conditions. From the decoupled IATE simulation, it is established that the boundary diameter must be used in the Nusselt number correlation in the condensation model to get good predictions of the interfacial area concentration. Moreover, the convection term in the IATE is found to contribute to interfacial area concentration at high mass flux conditions. The coupled IATE simulation model results in better agreement in the predictions of the void fraction and the interfacial area concentration under nearly all conditions with the proposed modifications to the condensation model. Sensitivity studies show the impact of Nusselt number correlation and gas velocity on the prediction of interfacial area concentration. Overall, a coupled simulation is recommended for benchmarking the IATE in phase change flows.

Experimental investigation on evaporation of R407C in a single horizontal smooth tube

Due to higher ozone layer depletion potential of HCFC refrigerant, R22 which has been mostly used in house hold refrigeration will be phased out by 2020 as per Montreal Protocol and UNFCCC Regulations. R407C, a zeotropic refrigerant from HFC category is a promising refrigerants in place of R22. Performance evaluation of R407 is required to enhance its application in house hold refrigeration. Hence an experimental investigation is carried out to understand the heat transfer characteristics during flow boiling of R407C in a smooth horizontal tube of 13.386 mm inner diameter and 2m length. The experiment is performed under the operating conditions; (i) mass flux range 100 to 300 kg s-1m-2; (ii) heat flux within range 2 to 7 kWm-2; (iii) temperature range at inlet to test section -100C to +100C; (iv) average vapor quality within test section from 0.05 to 0.95.The effect of heat flux, mass flux, vapor quality, temperature glide on heat transfer coefficient, during evaporation of R407C are examined.

Up-flow boiling of R1234yf in aluminum multi-port extruded tubes

The characteristics of local heat transfer and pressure drop were experimentally investigated for the vertical up-flow boiling of refrigerant R1234yf in two types of aluminum multi-port extruded tubes. One with 16 channels with a cross-section of 0.91×0.21mm (tube A), and one with 40 channels with a cross-section of 0.34×0.21mm (tube B). At an evaporating temperature of 15°C, the local heat transfer coefficient and pressure drop for the heat flux range of 3–16kW/m2, and mass flux range of 60–240kg/m2s were measured. The heat transfer coefficient decreased nearly linearly with increasing vapor quality in both tubes. As the vapor quality increased, the dry patch areas became larger in these rectangular channels, which degraded the heat transfer performance significantly. Compared with the multi-port extruded tube (tube C), with 16 channels with a cross-section of 0.9×0.9mm, at a mass flux of 60kg/m2s, tube C demonstrated better heat transfer performance than tubes A and B. When the mass flux increased to 120kg/m2s, the dry-out was alleviated in tubes A and B, and better performance was observed for low vapor quality regions, while in high vapor quality regions the dry patches reoccupied the heat transfer area, thereby resulting in worse heat transfer performance in tubes A and B. The pressure drop and heat transfer coefficient was not effectively predicted by the existing correlations.

Experimental investigation on the propagation characteristics of pressure oscillation in direct contact condensation with low mass flux steam jet

The propagation characteristics of pressure oscillation in direct contact condensation with low mass flux steam jet have been investigated experimentally. Steam is injected into subcooled water at one atmosphere pressure with steam mass flux and water temperature range of 186–272kg/(m2s) and 293–343K. The pressure oscillation propagates in the form of wave with stable dominant frequency, however the wave intensity attenuates with the increasing distance from the oscillation source. The root mean square of pressure wave prms attenuates rapidly with the increasing dimensionless radial distance from the nozzle exit. At about dimensionless radial distance R=100, the prms is attenuated by about 90%. Although the dominant frequency of the pressure oscillation is constant during the propagation, after R=100, there will be not enough energy for the pressure oscillation to resonate with relevant equipment. A correlation equation to calculate the root mean square of pressure oscillation along the radial distance is given. The prediction errors are within ±30% compared with the experimental data.

Pressure drop for highly subcooled water flow boiling under high heat and mass fluxes

Pressure drop is one of the most important thermal-hydraulic issues for the cooling system of high heat flux components, such as the plasma-facing divertors in fusion reactors. Experimental studies were carried out to investigate the pressure drop of highly subcooled water flow boiling in a vertical circular tube (d=9mm, LH=400mm) under high heat and mass fluxes. The operating parameters covered the range of heat fluxes (0–12.5MW/m2), mass fluxes (6000–10,000kg/m2s), pressures (3–5MPa), and liquid subcooling (6–224°C). The parameter effects on subcooled boiling pressure drop were discussed. It was observed that subcooled boiling pressure drop increased with increasing heat and mass fluxes but decreased with increasing pressure. Available correlations for subcooled boiling pressure drop were compared with the experimental data. The results indicate that these correlations cannot well predict our data mainly due to the variations in operating parameters and working fluids. Therefore, a new correlation is proposed to calculate subcooled boiling pressure drop for high-heat-flux applications. The new correlation is presented as a function of boiling number, Jacob number, and density ratio. The new correlation predicts the experimental data within an accuracy of ±20%.

Modeling of subcooled boiling by extending the RPI wall boiling model to ultra-high pressure conditions

Accurate predictions of the two-phase flow pattern and heat transfer inside the high-pressure tubes is of great importance for the safety of steam power equipment. In this paper, subcooled boiling flow was numerically modeled and the RPI (Rensselaer Polytechnic Institute) wall boiling model was extended to ultra-high pressure conditions up to 15MPa. Various combinations of closure models for the key parameters in RPI model, including the nucleation site density (Nw), bubble departure diameter (Dw) and bubble departure frequency (f), were studied in the pressure range of 3–15MPa. The numerical results were carefully validated by comparing the simulations with the experiments of Bartolemei et al. (1967, 1982). A widely applicable combination of closure models for Nw, Dw and f was identified for the pressure range from low to ultra-high levels, with the mass flux range of 503–2123kg/(m2s) and the heat flux range of 0.42–2.21MW/m2. The characteristic parameters of wall subcooled boiling were analyzed and the physical process of subcooled boiling was revealed. Based on the optimized models, the influence of operating parameters including mass flux and heat flux was also studied under the pressures of ∼11MPa and ∼15MPa.

Comparison and development of new correlation for adiabatic two-phase pressure drop of refrigerant flowing inside a multiport minichannel with and without fins

This study examines the experimental adiabatic two-phase frictional pressure drop of R134a in rectangular multiport minichannel with and without fins having 20 channels with hydraulic diameters of 0.64 mm and 0.81 mm, respectively. The pressure drop measurements were done under mass flux range of 50–200 kg m−2 s−1, saturation temperature range of 20–35 °C, and inlet vapor quality range of 0.1–0.9. The effects of mass flux, saturation temperature, inlet vapor quality and channel geometry on frictional pressure drop were investigated. The results discovered that the mass flux, inlet vapor quality, saturation temperature and channel geometry play an important role in increasing or decreasing the frictional pressure drop. The present experimental data were compared with eleven existing well known frictional pressure drop correlation available in the open literature. In addition, a new two-phase frictional pressure drop correlation is proposed considering the effects of inertia, viscous force, fluid properties, channel geometry and surface tension and also validated the correlation with the available data.

Flow boiling heat transfer and pressure drop analysis of R134a in a brazed heat exchanger with offset strip fins

The saturated flow boiling heat transfer and friction analysis of R 134a were experimentally analyzed in a brazed plate fin heat exchanger with offset strip fins. Experiments were performed at mass flux range of 50–82 kg/m2 s, heat flux range of 14–22 kW/m2 and quality of 0.32–0.75. The test section consists of three fins, one refrigerant side fin in which the boiling heat transfer was estimated and two water side fins. These three fins are stacked, held together and vacuum brazed to form a plate fin heat exchanger. The refrigerant R134a flowing in middle of the test section was heated using hot water from upper and bottom sides of the test section. The temperature and mass flow rates of water circuit is controlled to get the outlet conditions of refrigerant R134a. Two-phase flow boiling heat transfer and frictional coefficient was estimated based on experimental data for offset strip fin geometry and presented in this paper. The effects of mass flux, heat flux and vapour quality on heat transfer coefficient and pressure drop were investigated. Two-phase local boiling heat transfer coefficient is correlated in terms of Reynolds number factor F, and Martinelli parameter X. Pressure drop is correlated in terms of two-phase frictional multiplier ϕ f , and Martinelli parameter X.

Experimental investigation of refrigerant condensation heat transfer characteristics in the horizontal microfin tubes

The condensation heat transfer characteristics for refrigerant R22 and near-azeotropic mixture, R410A are investigated in the horizontal smooth tube and microfin tube, respectively. The 9.52-mm-OD tube with active length 3.4m and 5-mm-OD tube with 5m are selected for condensation test cooled by the heat transfer fluid (water) circulated in a surrounding annulus. A refrigerant superheating 4–7°C is set at the test section inlet and the mass flux range is 100–400kgm−2s−1 for 9.52-mm-OD tube, and 300–550kgm−2s−1 for the 5-mm-OD tube. The heat transfer coefficient (HTC) of the microfin tube is around 1.65–2.55 times of that of the smooth one. The microfin one yields ∼30% higher pressure drop than the smooth one. A lower condensation saturation temperature can generate a higher HTC and a higher pressure drop. 5-mm-OD tube produces both the higher HTC and pressure drop than the 9.52-mm-OD one due to the increased effect of the surface tension and the sheer stress at the gas-liquid interface. R22 performs marginally better for HTC than R410A and the former has a ∼40% higher pressure drop. In addition, it is necessary to collect more data source and make a generally-accepted and more accurate correlation in the near future.

A self-consistent, physics-based boiling heat transfer modeling framework for use in computational fluid dynamics

Computational Fluid Dynamics (CFD) offers the opportunity to investigate physically and geometrically complex systems with high fidelity. Its applicability to multiphase flow, and particularly boiling heat transfer, is currently limited by the lack of appropriate closure models to describe all relevant phenomena. In this paper, we present an original subcooled flow boiling modeling framework for CFD, which aims at consistently and accurately characterizing the key physics that affect heat transfer at the boiling surface. The new framework introduces a fully mechanistic representation of heterogeneous boiling that improves numerical robustness and reduces sensitivity to closure coefficients. The proposed formulation is inspired by new experimental insight, and significantly extends the existing boiling models by capturing the effects of (i) the microlayer on surface evaporation, (ii) the boiling surface, and (iii) bubbles sliding along the boiling surface. A new statistical treatment of the location and mutual interactions of bubbles on the surface allows for mechanistic prediction of the dry surface area, an important quantity that affects the boiling heat transfer coefficient. This approach lends itself naturally to extension to very high heat fluxes, potentially up to the critical heat flux. An assessment and sensitivity study of the model is presented for a range of mass fluxes (500–1250kg/m2/s), heat fluxes (100–1600kW/m2), inlet subcoolings (5, 10, 15K), and pressures (1, 1.5, 2 bars), demonstrating improved robustness and predictive accuracy at all tested conditions in comparison to traditional heat partitioning approaches, including high heat fluxes, where classic models often fail to converge. Lastly, the framework proposed here should not be viewed as another heat partitioning model, but rather as a general platform that allows incorporation of advanced models for each physical phenomenon considered, leveraging the growing insight generated by modern experimental diagnostics for boiling heat transfer.

Boiling Heat Transfer of R32, CO2 and R290 inside Horizontal Minichannel

This study demonstrated two phase flow boiling heat transfer coefficient of R32 (difluoromethane), CO2 (carbon dioxide) and R290 (propane) in minichannel. The experimental data were conducted in horizontal stainless steel tubes with the inner diameter of 1.5 mm. The testing conditions were performed with the mass fluxes range from 150 to 500 kg/m2s, the heat fluxes from 5 to 20 kW/m2, the saturation temperature of 10°C and the vapor quality from 0.1 to dry-out. The effects of mass flux and heat flux on the heat transfer coefficient were analyzed. The experimental data were also compared with some well-known heat transfer coefficient correlations.

Convective heat transfer at high subcritical pressures in tubes with and without flow obstacles

Heat transfer measurements to CO2-cooled tubes with and without flow obstacles at high subcritical pressures were obtained at the University of Ottawa’s supercritical test facility. Tests were performed for vertical upward flow in a directly heated 8mm ID tube for a pressure range from 6.6 to 7.2MPa, a mass flux range from 115 to 1000kgm−2s−1, and a wall heat flux range from 1 to 122kWm−2. Cases with spacing between obstacles equal to 250, 500 and 1000mm were investigated. The results are presented in plots of wall temperature and heat transfer coefficient vs. bulk fluid enthalpy. The effects of flow conditions and obstacle geometry and spacing on high subcritical heat transfer have been discussed. Obstacles were found to suppress nucleate boiling, increase the CHF and lower the maximum post-CHF temperature. The maximum increases in CHF and in post-dryout heat transfer coefficient were observed for the obstacles having a spacing of 250mm. Comparisons of the experimental data with prediction methods for single-phase heat transfer, CHF and post-dryout heat transfer are also presented.

An empirical correlation for the entrainment fraction at the onset of annular flow based on 2006 CHF look-up table

The critical heat flux (CHF) of gas-liquid flow plays an important role in the safety of industrial equipment. At present, the liquid film dryout model is widely used for predicting CHF in gas-liquid annular flow. Most parameters in this model can be determined by some empirical correlations which are valid under different conditions. However, up to now, the entrainment fraction at the onset of annular flow is always assumed due to the lack of relevant experimental data. In this paper, the normalized data of the 2006 CHF look-up table (LUT) which has been adopted widely, especially in the nuclear industry, were used. Firstly, the empirical correlations, provided for the onset of annular flow and the limiting quality, were employed. In the valid pressure and mass flux range of these correlations, the selected database from LUT was confirmed. Secondly, the liquid film model was built. The entrainment fraction at the onset of annular flow was obtained when the calculated CHF by the model agreed with the corresponding value in LUT. A parallel look-up table for it was developed. Its correlation including the Weber and the liquid Reynolds number at outlet was proposed. The errors are mostly within ±30%. Finally, its reliability on the other conditions in LUT, which are beyond the valid range of the empirical correlations used for determining the database, was discussed. All the conditions whose errors are outside ±30% of the predictions by the provided correlation were marked in the tables.