Increasing Mass Velocity Research Articles

Critical heat flux of cyclohexane in uniformly heated minichannels with high inlet subcooling

The critical heat flux (CHF) for flow boiling of cyclohexane was experimentally investigated in electrically heated minichannels with inside diameters of (1 and 2)mm, and heated lengths of (360 and 710)mm at mass velocities ranging from (318 to 1274)kg/m2s, heat fluxes from (50 to 1000)kW/m2, and pressures of (1.0, 2.0, 3.0)MPa. Cyclohexane entered the test channel at ambient temperature of about 20°C, equivalent to high subcooling of (162.1, 206.4 and 236.3)°C at (1.0, 2.0 and 3.0)MPa respectively. The critical vapor qualities ranged from −1.35 to 0.83. Results indicated that CHF of cyclohexane increased with increasing mass velocity and decreasing heated length. Reducing channel diameter at the same length-to-diameter ratio was proved to have no effects on CHF values of cyclohexane at the evaluated conditions. CHF mildly increased with an increase in pressure at higher mass velocities, but critical vapor quality always decreased with the increasing pressure. Experimental CHF data were compared with the Katto–Ohno (1984) correlation and Shah (1987) correlation. Good agreements were achieved between the experimental data and those predicted with most deviations not exceeding 30%. A corrected Katto–Ohno CHF correlation is developed to fit the current experimental data. Excellent agreement was obtained with all the deviations (whatever of saturation CHF or subcooled CHF) in the range of ±10% with RMS error of 5.35%.

ВИМІРЮВАННЯ І МОДЕЛЮВАННЯ КОЕФІЦІЄНТА ТЕПЛОВІДДАЧІ ПРИ КИПІННІ РОЗЧИНІВ R600a/КОМПРЕСОРНЕ МАСТИЛО ВНУТРІ ГОРИЗОНТАЛЬНОЇ ГЛАДКОЇ ТРУБИ

Experimental results of local heat transfer coefficients for the boiling of real working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in smooth tube with small diameter (5.4 mm) are presented. The tests were carried out for the inlet pressure in the range from 71.1 kPa to 77.9 kPa, heat flux was 3800 W/m2, and mass velocity of working fluid was from 14.75 to 18.36 kg/(m2s). The quantitative estimation in reduction of heat transfer coefficient of the wetted surface in evaporator at high oil concentration in the mixture is made. The influence of mass velocities of the working fluid on the values of the local heat transfer coefficients are analyzed. Based on the results obtained it was observed that increasing mass velocity leads to increase the local heat transfer coefficient of RWF both on side of wetted perimeter and vapor phase. The equation for the modeling of the local heat transfer coefficient for boiling of isobutane/compressor oil solution flow in the pipe is suggested.

Flow boiling heat transfer of R407C in a microchannels based heat spreader

New flow boiling experimental results for R407C in a microchannel based heat spreader are presented. Boiling curves were obtained for heat fluxes up to 310kW/m2 (based on the footprint area), mass velocities from 400 to 1500kg/m2s, liquid subcoolings at the test section inlet of 5, 10 and 15°C and saturation temperatures referred to the pressure at the heat sink inlet of approximately 25°C. Based on these results, heat sink averaged heat transfer coefficients during convective boiling were estimated. The heat sink evaluated in the present study is comprised of fifty parallel rectangular channels with cross-section dimensions of 100×500μm, and total length of 15mm. Average heat transfer coefficients up to 30kW/m2°C were obtained. It was also found that the boiling curve moves to the left hand side with decreasing the mass velocity and liquid subcooling at the heat-sink inlet. Moreover, for a fixed heat-sink averaged vapor quality, the average heat transfer coefficient increases with increasing mass velocity. Under similar experimental conditions, the refrigerant R134a provided higher heat transfer coefficients than R407C. Additionally, during flow boiling of R407C, pressure oscillations with lower amplitude and frequency were observed compared to R134a. No one of the heat transfer predictive methods evaluated in the present study was accurate enough to predict the present R407C database.

Critical Heat Flux of R134a and R245fa Inside Small-Diameter Tubes

This article presents new experimental critical heat flux results under saturated flow boiling conditions for a macro-/microscale tube. The data were obtained in a horizontal 2.20-mm inside diameter stainless-steel tube with heating lengths of 361 and 154 mm, R134a and R245fa as working fluids, mass velocities ranging from 100 to 1500 kg/m2-s, critical heat flux from 25 to 300 kW/m2, exit saturation temperatures of 25, 31, and 35°C, and critical vapor qualities ranging from 0.55 to 1. The experimental results show that critical heat flux (CHF) increases with increasing mass velocity and inlet subcooling but decreases with increasing saturation temperature and heated length. The data also indicated a higher CHF for R245fa when compared with R134a at similar conditions. The experimental data were compared against four CHF predictive methods and the results of the comparisons are reported.

An experimental study on flow boiling heat transfer of R134a in a microchannel-based heat sink

The present study reports on an experimental evaluation of a heat-sink based on flow boiling of R134a inside micro-scale channels. The heat-sink is comprised of fifty parallel rectangular microchannels with cross-sectional dimensions of 100μm width and of 500μm depth, and total length of 15mm. The fins between consecutive microchannels are 200μm thick. Experiments were performed for R134a, heat fluxes up to 310kW/m2 (based on the foot print area), mass velocities ranging from 400 to 1500kg/m2s and saturation temperatures approximately 25°C at the microchannels outlet. Heat-sink averaged heat transfer coefficients based on the heated area up to 36kW/m2°C were obtained. The effect of mass velocity on the wall superheating excess necessary for the onset of nucleate boiling is not clear under low mass velocity conditions, however it is negligible for mass velocities higher than 1000kg/m2s. The boiling curves displaying the heat flux versus average heat sink superheating are shifted to the right hand side with increasing mass velocity and fluid subcooling at the heat-sink inlet. Keeping the average vapor quality over the heat-sink fixed, the average heat transfer coefficient increases when the mass velocity increases. The three-zones model by Thome and coworkers provided reasonable predictions of the present experimental results and captured most of the heat transfer trends.

Heat sensitivity of vertical water wall at low mass velocity in supercritical pressure W-shaped flame boiler

The vertical water wall of the once-through boiler is sensitive to heat disturbance because of its smaller water volume compared with the drum boiler. This paper deduced formulas of the sensitivity coefficient of flow and output enthalpy, and made a heat sensitivity analysis for the vertical water wall of a 600 MW supercritical pressure W-shaped flame boiler at a low mass velocity when heat disturbance occurs. It was found that the flow sensitivity coefficient decreases and the outlet enthalpy sensitivity coefficient increases with increasing steam quality. Increasing mass velocity weakens the flow sensitivity and enhances the outlet enthalpy sensitivity. When the mass velocity exceeds a certain value, the flow sensitivity coefficient is less than zero. Increasing heat flux leads to an increase in the flow sensitivity coefficient and a decrease in the outlet enthalpy sensitivity coefficient. In the condition of low steam quality, the flow sensitivity weakens and outlet enthalpy sensitivity strengthens when the pressure is up to 22 MPa. Moreover, when the quality is high, rising pressure strengthens the flow sensitivity and weakens the outlet enthalpy sensitivity. The study has provided some instructive findings, which may be helpful for the safety operation of the 600 MW supercritical pressure W-shaped flame boiler.

Experimental Study on Condensation of R134a inside Horizontal Inner-Micro-Fin Tubes

An experimental study of condensation heat transfer of R134a on horizontal inner enhanced tubes was conducted. The tested tubes were inner-micro-fin tubes, named tube A and tube B, respectively. The tested pieces were double-pipe condensers. The glycol solution flowed in the space between outer surface of the enhanced tube and inner surface of outer tube. In the experiment, condensing temperature inside the enhanced tube was 51°C, and the flow velocity of glycol solution was 3.35m/s. The inlet temperature of glycol solution changed according to mass velocity of refrigerant, to maintain certain degree of undercooling of outlet refrigerant. The research showed that the condensation heat transfer coefficient of both tubes increased with the increasing mass velocity of refrigerant. when the mass velocity of refrigerant increased from 300kg/m2s to 700kg/m2s, the condensation heat transfer coefficient in tube A was 1.87% to 6.28% higher than that of tube B. However, the flow resistance of the refrigerant in tube B was 9.56% to 11.05% higher than in tube A. The structure of tube A was superior to that of tube B.

사다리꼴 미세유로의 대류비등 2상유동 : 1부-압력강하 특성

Characteristics of two-phase pressure drop in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having of bottom width, of depth, of sidewall angle, and 7 cm of length. Pressure drops in convective boiling of Refrigerant 113 were measured in the range of inlet pressure 105~195 kPa, mass velocity , and heat flux . The total pressure drop generally increased with increasing mass velocity and/or heat flux. Two-phase frictional pressure drop across the microchannels increased rapidly with exit quality and showed bigger gradient at higher mass velocity. A critical review of correlations in the literature suggested that existing correlations were not able to match the experimental results obtained for two-phase pressure drop associated with convective boiling in microchannels. A new correlation suitable for predicting two-phase friction multiplier was developed based on the separated flow model and showed good agreement with the experimental data.

Evaluation of flow patterns and elongated bubble characteristics during the flow boiling of halocarbon refrigerants in a micro-scale channel

In the present study, quasi-diabatic two-phase flow pattern visualizations and measurements of elongated bubble velocity, frequency and length were performed. The tests were run for R134a and R245fa evaporating in a stainless steel tube with diameter of 2.32 mm, mass velocities ranging from 50 to 600 kg/m 2 s and saturation temperatures of 22 °C, 31 °C and 41 °C. The tube was heated by applying a direct DC current to its surface. Images from a high-speed video-camera (8000 frames/s) obtained through a transparent tube just downstream the heated sections were used to identify the following flow patterns: bubbly, elongated bubbles, churn and annular flows. The visualized flow patterns were compared against the predictions provided by Barnea et al. (1983) [1], Felcar et al. (2007) [10], Revellin and Thome (2007) [3] and Ong and Thome (2009) [11]. From this comparison, it was found that the methods proposed by Felcar et al. (2007) [10] and Ong and Thome (2009) [1] predicted relatively well the present database. Additionally, elongated bubble velocities, frequencies and lengths were determined based on the analysis of high-speed videos. Results suggested that the elongated bubble velocity depends on mass velocity, vapor quality and saturation temperature. The bubble velocity increases with increasing mass velocity and vapor quality and decreases with increasing saturation temperature. Additionally, bubble velocity was correlated as linear functions of the two-phase superficial velocity.

Critical heat flux for subcooled flow boiling in micro-channel heat sinks

Critical heat flux (CHF) was measured and examined with high-speed video for subcooled flow boiling in micro-channel heat sinks using HFE 7100 as working fluid. High subcooling was achieved by pre-cooling the working fluid using a secondary low-temperature refrigeration system. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid, which is consistent with the mechanism of Departure from Nucleate Boiling (DNB). CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink’s inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels. This technique is shown to be successful at correlating micro-channel heat sink data corresponding to different hydraulic diameters, mass velocities and inlet temperatures.

Elongated bubbles in microchannels. Part I: Experimental study and modeling of elongated bubble velocity

The velocity of elongated vapor bubbles exiting two horizontal micro-evaporator channels with refrigerant R-134a was studied. Experiments with tube diameters of 509 and 790 μm, mass velocities from 200 to 1500 kg/m 2 s, vapor qualities from 2% to 19% and a nominal saturation temperature of 30 °C were analyzed with a fast, high-definition digital video camera. It was found from image processing of numerous videos that the elongated bubble velocity relative to that of homogeneous flow increased with increasing bubble length until a plateau was reached, and also increased with increasing channel diameter and increasing mass velocity. Furthermore an analytical model developed for a diabatic two-phase flow, has been proposed that is able to predict these trends. In addition, the model shows that the relative elongated bubble velocity should decrease with increasing pressure, which is consistent with the physics of two-phase flow.

Heat Transfer Analysis of Different Storing Media Using Oil as Working Fluid

Thermal analysis of heat transfer through different storing media using oil as working fluid is presented. The storing medium is solid material in spherical shape. Steel, glass, and pebbles are selected as storing media and oil is selected as working fluid. The physical model is a heat exchanger in cylindrical shape, which is packed with each of the selected storing medium. The heat transfer through the heat exchanger is assumed to be one dimensional along its height. The flow of the working fluid is an axial direction from the top to downward. The problem is governed by two partial differential equations for the working fluid (oil) and the storing medium. Finite difference method and Thomas algorithm solver are used to solve the couple of the two partial differential equations along with their associated initial and boundary conditions. The modified computer program is used to obtain the solution of transient temperature distribution of the storing medium and the working fluid. The amount of absorbed heat inside each of the storing medium is obtained. The effect of special operating parameters on the amount of absorbed heat inside the storing medium, such as aspect ratio (the ratio between diameter and length of the heat exchanger), storing media, mass velocity, the number of charging cycles, and void fraction, is discussed. Therefore, the dimensionless heat transfer coefficient parameter (Nusselt number, Nu) provides a measure of the convection and conduction heat transfer at the surface of storing medium when the working fluid (oil) flows over a solid surface of the medium. The numerical results of transient temperature profiles and the amount of absorbed heat inside the storing medium for each system with respect to the operating parameters and the heat exchanger characteristics are illustrated. The results show that steel storing medium is charging by four cycles while the pebble storing medium is charging by two cycles only, this due to the thermal and physical properties of these materials. The absorbed heat inside storing medium, which has aspect ratio equals one (diameter of the heat exchanger equals its length) is higher than others. Increasing mass velocity increases absorbed heat inside the storing medium and decreasing the charging time. Increasing void fraction decreases absorbed heat inside the storing media due to the smaller volume of absorbing medium. The amount of absorbed heat (at certain time) inside the steel > glass > pebble is due to the thermal conductivity of these materials.

Measurement and correlation of critical heat flux in two-phase micro-channel heat sinks

Critical heat flux (CHF) was measured for a water-cooled micro-channel heat sink containing 21 parallel 215 × 821 μm channels. Tests were performed with deionized water over a mass velocity range of 86–368 kg/m 2s, inlet temperatures of 30 and 60 °C, at an outlet pressure of 1.13 bar. As CHF was approached, flow instabilities induced vapor backflow into the heat sink’s upstream plenum, which significantly altered the coolant temperature at the channel inlets. The backflow negated the advantages of inlet subcooling, resulting in a CHF virtually independent of inlet temperature but which increases with increasing mass velocity. Due to the vapor backflow and other unique features of parallel micro-channels, it is shown previous correlations that are quite accurate at predicting CHF for single mini-channels are unsuitable for micro-channel heat sinks. Using the new heat sink water CHF data as well as previous data for R-113 in heat sinks with multiple circular mini- and micro-channels, a new CHF correlation is proposed which shows excellent accuracy in predicting existing heat sink data.

Ultra-high critical heat flux (CHF) for subcooled water flow boiling—I: CHF data and parametric effects for small diameter tubes

Ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m −2, were obtained using high mass velocity, subcooled water flow through short, small diameter tubes. These tests produced the highest CHF of 276 MW m −2 reported in the literature for a uniformly heated tube which surpassed the prior record of 228 MW m −2. The data include broad ranges of tube diameter (0.406–2.54 mm) , heated length-to-diameter ratio (2.4–34.1) , mass velocity (5000–134 000 kg m −2 s −1) , inlet temperature (18–70°C) , and outlet pressure (2.5–172.4 bars) . The parametric trends of CHF were ascertained relative to all important flow and geometrical parameters. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length-to-diameter ratio. For a constant inlet temperature, CHF increased with increasing pressure for pressures up to 30 bars, remained fairly constant between 30 and 150 bars, and decreased afterwards as the critical pressure was approached. CHF was accompanied by physical burnout of the tube wall near the exit and tube material had little effect on the magnitude of CHF. The pressure drop for most conditions was fairly constant, albeit as high as 153.4 bars, over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving that CHF was triggered even with negligible net vapor production. These high pressure drops indicate special attention should be exercised when employing the high mass velocity flows necessary to attaining ultra-high CHF. These high pressure drops also render the practice of referencing CHF data reSlative to a single measured pressure value very misleading.

Friction pressure drop of R-12 in small hydraulic diameter extruded aluminum tubes with and without micro-fins

Adiabatic, single-phase liquid and two-phase flow pressure drop were measured for R-12 flowing in both rectangular plain and micro-fin tubes with hydraulic diameters 2.64 and 1.56 mm, respectively. The single-phase liquid friction factors for the plain and micro-fin tubes are uniformly 14% and 36% higher, respectively, than that predicted by the Blasius equation. For two-phase flow, the pressure gradient increases with increasing mass velocity and vapor quality. The pressure gradient of the micro-fin tube is higher than that of the plain tube at same mass velocity and vapor quality. Predictive methods for the single-phase liquid and two-phase friction factor were also developed. These data are not well correlated by the Chisholm correlation which uses the Lockhart-Martinelli two-phase multiplier. However, the equivalent mass velocity concept proposed by Akers et al. provided a very good correlation of the present data. Both the plain and micro-fin tube data are correlated within ±20% by a single curve.This work shows that the pressure drop is dominated by vapor shear in both the plain and micro-fin tubes. Vapor shear effects in micro-fin tube do not cause significant disturbances in the two-phase flow. This observation provides additional evidence to support the conclusion in other work by Yang and Webb that the distinctly steep condensation heat transfer curves at low mass velocity and high vapor quality are caused by surface tension drainage force.

High-quality critical heat flux in horizontally coiled tubes

An investigation on the high-quality dryout in two electrically heated coiled tubes with horizontally helix axes is reported. The temperature profiles both along the tube and around the circumference are measured, and it is found that the temperature profiles around the circumference are not identical for the cross-sections at different parts of the coil. The “local condition hypothesis” seems applicable under present conditions, and the critical heat fluxqcrdecreases with increasing critical qualityxcr. The CHF increases as mass velocity and ratio of tube diameter to coil diameter (d/D) increases, and it seems not to be affected by the system pressure. The CHF is larger with coils than that with straight tubes, and the difference increases with increasing mass velocity andd/D.

Two‐phase pressure drop in vertical crossflow across a horizontal tube bundle

AbstractAn experimental investigation has been made to evaluate the friction, acceleration, and hydrostatic pressure drops in two‐phase vertical crossflow across a horizontal tube bundle through the measurement of the void fraction and determination of the two‐phase friction multiplier. The void fractions were found to increase with increasing mass velocity for a fixed quality level. The two‐phase friction multiplier increased with increasing mass velocity for a fixed value of the Martinelli parameter in both slug and spray flow and decreased with increasing mass velocity in bubbly flows. The void fraction and two‐phase friction multiplier data were correlated and used to predict with very good results the total pressure drop occurring in simulated diabatic flow tests and in actual diabatic tests using R‐113.

Boiling Heat Transfer From a Horizontal Tube in an Upward Flowing Two-Phase Crossflow

An experimental investigation has been performed to determine the effects of a low-quality (≤ 20 percent) upward flowing mixture on the nucleate boiling on a single horizontal lube. An electrically heated, 12.7-mm-dia tube was centered in a plane wall vertical channel, the width of which resulted in channel width-to-tube diameter ratios (w/d) of 1.16 and 1.95. The working fluid was R-113. The two-phase heat transfer data showed a variety of effects. For a fixed w/d, pressure (P), and quality (x), the average heat transfer coefficients (h) increased with increasing mass velocity (G), but the effect of G decreased as the wall superheat (ΔT) increased. For a fixed w/d, G and x, h increased as the pressure increased except at low ΔT’s where the reverse was found. For fixed w/d, P and G, h increased with increasing quality with the effect appearing to be more pronounced at the lower pressure. At a fixed P, G and x, h was at larger w/d ratios at small ΔT’s, but as the wall superheat increased an inversion occured and h became smaller at the larger w/d ratio. The behavior exhibited in this experiment can be explained in terms of the velocity of the fluid flowing past the test section. The data were successfully predicted to within an average deviation of ±11.6 percent using a Chen-type correlation. Data from the literature also were predicted well.

Critical Heat Flux in Helically Coiled Tubes

A study of boiling R-113 in electrically heated coils of various diameters is reported. Subcooled critical heat flux (CHF) is lower with coils than with straight tubes. The difference increases as mass velocity and ratio of tube diameter to coil diameter (d/D) increases. On the contrary, quality CHF is enhanced and increases with d/D; CHF initially increases with increasing mass velocity, but decreases after a maximum is reached. Operational problems, in particular upstream dryouts, can occur if a coiled tube is operated with low to moderate subcooling near the inlet and with moderately high heat fluxes.

A study of slip ratios for the flow of steam‐water mixtures at high void fractions

AbstractA technique employing the impulse plate principle was developed whereby the ratio of the average vapor‐to‐liquid velocities (slip ratio) for flowing two‐phase mixtures could be measured accurately at high vapor volume fractions. Data were collected for steam‐water mixtures flowing adiabatically in a horizontal ½‐in. tube. Flow conditions were in the spray annular and dispersed flow regimes and covered a pressure range of 30 to 80 lb.f/sq.in.abs., flow rates of 200 to 800 lb.m/(sec.)(sq.ft.), and steam qualities of 0.02 to 0.8. The experimental slip ratios, ranging between 1 and 3.5, decreased with increasing quality and pressure and increased with increasing mass velocity and pressure gradient.A theoretical analysis in which an idealized dispersed flow model was used indicated that the observed average slip ratios were caused largely by local slip between vapor and entrained droplets and that high local slip ratios may be attained near critical flow rates due to the simultaneously occurring steep pressure gradients.The total pressure gradients, computed by adding the Martinelli‐Nelson frictional pressure drop prediction to the acceleration pressure gradients calculated by the use of an empirical correlation of the slip ratio data, deviated from the experimental values by an average of only 14%.