Saturated Boiling Heat Transfer Coefficients Research Articles

Experimental study of flow boiling characteristics in minigap channels over a wide heat flux range

The flow boiling characteristics in minigap channels were investigated by observing the two-phase flow patterns, measuring the two-phase pressure drops, and calculating the two-phase heat transfer coefficients and critical heat fluxes in minigap channels with heights of 0.5–2 mm. The working fluid was deionized water with mass fluxes of 200–400 kg m−2 s−1. The results show that bubbly flow, sweeping flow, churn flow, and churn-annular flow occur in sequence with increasing heat fluxes. An unstable two-phase flow regime was observed during the sweeping flow. The pressure drop varied in three regimes that were related to the flow patterns. The maximum pressure drops are 12.2, 3.8 and 1.6 kPa, respectively in 0.5 mm, 1 mm and 2mm channels at 400 kg m−2 s−1. The effect of gap height on the heat transfer coefficient first increases and then decreases with increasing heat flux. The heat transfer coefficients of all channels are at a high level when the heat flux is high. The maximum heat transfer coefficients are 7.3, 6.9 and 5.8 W cm−2 K−1 in the 0.5 mm, 1mm and 2mm channels at 400 kg m−2 s−1, which occurred before critical condition. The critical heat flux increased with increasing mass flux and gap height. In addition, a pressure drop correlation was developed to more accurately predict the pressure drop in microgap channels with an accuracy of 10.8%. The Liu-Winterton correlation and the Shah correlation give the best predictions for the subcooling boiling heat transfer coefficient. The Sun-Mishima correlation and the Kandlikar correlation provide the most accurate predictions for the saturated boiling heat transfer coefficients.

Experimental investigation on oil transport, heat transfer and distribution performances of R32/oil mixture in microchannel evaporators

The aim of this study is to experimentally investigate the oil influences on transport, heat transfer and distribution performances of microchannel evaporator. Mixture of R32 and PVE VG68 oil was selected as the working fluid, and two single-pass microchannel heat exchangers were used as test samples. The experiments were conducted in mass flux range of 20–50 kg·m−2·s−1, nominal oil mass fraction (OMFno) range of 0–6% and inlet local quality range of 0.1–0.16 at the saturation temperature of 12 °C. The result indicated that the oil retention volume ratio (ORVR) increases with the increments in oil mass fraction, inlet quality and decrease in mass flux. The upper outlet header tended to have remarkable oil retention and the oil transport behavior in header was analyzed. In most cases, the heat transfer was suppressed by the addition of oil. Heat transfer enhancement was observed at low oil mass fraction and high inlet quality conditions. The heat transfer factor ranged from 0.95 to 1.03, which indicated that the oil effect on heat transfer was not significant. Although the presence of oil inhibited the saturated boiling heat transfer coefficient, it could promote the heat transfer at superheat region. Struggle of inhibition and enhancement effects resulted in slight oil effect on heat transfer performance. An infrared-image-based distribution rating parameter was utilized to quantify the distribution performance. The oil effect on flow distribution was slight and the flow distribution deteriorated as the mass flux increased. A theoretical model called liquid immersion model was adopted to describe the flow regime and distribution mechanism in inlet header.

A relatively wide-ranged correlation of saturated flow boiling heat transfer within narrow rectangular channel for water

• Database of boiling heat transfer in narrow rectangular channel over a relatively wide parametric range obtained. • Corresponding transition of two-phase flow regimes with dominant heat transfer mechanisms observed and discussed. • A new, wide-ranged correlation proposed. A campaign of tests is performed for saturated boiling heat transfer characteristics of water in narrow rectangular channel. The test matrix covers a relative wide range of conditional parameters and 3142 test data are acquired, screened and selected for study. Meanwhile, visualization test has been conducted with three typical flow patterns, namely isolated bubbly, confined bubbly and irregular annular flows, being observed and identified, each pattern corresponding to a prevailing or a transitional combined boiling heat transfer mode. Study of the data trend indicates that two boiling heat transfer mechanisms, namely nucleate boiling and forced convective evaporation, are either one dominant or two combined transitional during different phases of saturated flow boiling within the channel. With the basic view obtained from the test campaign, and with related parametric effect and sensitivity study, as well as the existing correlation assessment with the test data, a new correlation for the saturated boiling heat transfer is proposed and regressed, which consists a modification factor composing the two basic boiling mechanisms and the transition function between. A preliminary assessment is performed for its statistical and predictive accuracy, from which a relatively satisfactory agreement is drawn. The new correlation applies to a heat flux range of 100 - 2300 k W / m 2 , a mass flux range of 500 2000 k g / m 2 · s , a system pressure range of 0.2 5.5 M P a and the equilibrium quality range of 0 0.72 . It might be used, in conjunction with the flow pattern and the related heat transfer mechanism, to predict the saturated boiling heat transfer coefficient over a wide range of heat flux, mass flux and pressures with better accuracy than previous correlations.

Boiling of the new low-GWP refrigerants R1234ze(Z) and R1233zd(E) inside a small commercial brazed plate heat exchanger

This paper presents the heat transfer coefficients and the frictional pressure drops of R1234ze(Z) and R1233zd(E) boiling inside a commercial Brazed Plate Heat Exchanger (BPHE): the effects of heat flux / mass flux, saturation temperature / pressure, outlet conditions and fluid properties are investigated. The boiling heat transfer coefficients are controlled mainly by heat flux / mass flux and evaporator outlet conditions. The degrees of superheating at the outlet of the evaporator produces a degradation of the average boiling heat transfer coefficient in the whole evaporator, particularly for R1234ze(Z). The frictional pressure drops exhibit a quadratic dependence on refrigerant mass flux. Saturation temperature / pressure has a remarkable influence only on the frictional pressure drops. R1234ze(Z) exhibits boiling heat transfer coefficients higher and frictional pressure drops lower than those of R1233zd(E). There is a reasonable agreement between the saturated boiling heat transfer coefficients and the calculated values by two recent models for boiling inside BPHE.

HFC32 vaporisation inside a Brazed Plate Heat Exchanger (BPHE): Experimental measurements and IR thermography analysis

This paper presents HFC32 average boiling heat transfer coefficients and pressure drops measured inside a small Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, saturation temperature (pressure), and outlet conditions are investigated. The experimental tests were carried out at four different saturation temperatures (5, 10, 15, and 20 °C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10 °C). The average heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature (pressure). The saturated boiling heat transfer coefficients were compared with a new model for refrigerant vaporisation inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 4.7%. The heat transfer and pressure drop measurements are complemented with a IR thermography analysis for a better understanding of the vaporisation process inside a BPHE.

Surface Roughness Effects on Flow Boiling in Microchannels

The influence of surface roughness on flow boiling heat transfer and pressure drop in microchannels is experimentally explored. The microchannel heat sink employed in the study consists of ten parallel, 25.4 mm long channels with nominal dimensions of 500×500 μm2. The channels were produced by saw-cutting. Two of the test piece surfaces were roughened to varying degrees with electrical discharge machining (EDM). The roughness average Ra varied from 1.4 μm for the as-fabricated, saw-cut surface to 3.9 μm and 6.7 μm for the two roughened EDM surfaces. Deionized water was used as the working fluid. The experiments indicate that the surface roughness has little influence on boiling incipience and only a minor impact on saturated boiling heat transfer coefficients at lower heat fluxes. For wall heat fluxes above 1500 kW/m2, the two EDM surfaces (3.9 μm and 6.7 μm) have similar heat transfer coefficients that were 20–35% higher than those measured for the saw-cut surface (1.4 μm). A modified Bertsch et al. [2009, “A Composite Heat Transfer Correlation for Saturated Flow Boiling in Small Channels,” Int. J. Heat Mass Transfer, 52, pp. 2110–2118] correlation was found to provide acceptable predictions of the flow boiling heat transfer coefficient over the range of conditions tested. Analysis of the pressure drop measurements indicates that only the roughest surface (6.7 μm) has an adverse effect on the two-phase pressure drop.

Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays

Flow boiling in arrays of parallel microchannels is investigated using a silicon test piece with imbedded discrete heat sources and integrated local temperature sensors. The microchannels considered range in width from 102 μm to 997 μm, with the channel depth being nominally 400 μm in each case. Each test piece has a footprint of 1.27 cm by 1.27 cm with parallel microchannels diced into one surface. Twenty five microsensors integrated into the microchannel heat sinks allow for accurate local temperature measurements over the entire test piece. The experiments are conducted with deionized water which enters the channels in a purely liquid state. Results are presented in terms of temperatures and pressure drop as a function of imposed heat flux. The experimental results allow a critical assessment of the applicability of existing models and correlations in predicting the heat transfer rates and pressure drops in microchannel arrays, and lead to the development of models for predicting the two-phase pressure drop and saturated boiling heat transfer coefficient.

Saturated flow boiling heat transfer and pressure drop of refrigerant R-410A in a vertical plate heat exchanger

Saturated flow boiling heat transfer and the associated frictional pressure drop of the ozone friendly refrigerant R-410A (a mixture of 50 wt% R-32 and 50 wt% R-125) flowing in a vertical plate heat exchanger (PHE) are investigated experimentally in the study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of saturated refrigerant R-410A in one channel receives heat from the downflow of hot water in the other channel. The experimental parameters in this study include the refrigerant R-410A mass flux ranging from 50 to 125 kg/m 2 s and imposed heat flux from 5 to 35 kW/m 2 for the system pressure fixed at 1.08, 1.25 and 1.44 MPa, which respectively correspond to the saturated temperatures of 10, 15 and 20 °C. The measured data showed that both the boiling heat transfer coefficient and frictional pressure drop increase almost linearly with the imposed heat flux. Furthermore, the refrigerant mass flux exhibits significant effect on the saturated flow boiling heat transfer coefficient only at higher imposed heat flux. For a rise of the refrigerant pressure from 1.08 to 1.44 MPa, the frictional pressure drops are found to be lower to a noticeable degree. However, the refrigerant pressure has very slight influences on the saturated flow boiling heat transfer coefficient. Finally, empirical correlations are proposed to correlate the present data for the saturated boiling heat transfer coefficients and friction factor in terms of the Boiling number and equivalent Reynolds number.

Traction engines or road locomotives: The past, the present and the future of steam on the common road

This paper presents HFC32 average boiling heat transfer coefficients and pressure drops measured inside a small Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, saturation temperature (pressure), and outlet conditions are investigated. The experimental tests were carried out at four different saturation temperatures (5, 10, 15, and 20 °C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10 °C). The average heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature (pressure). The saturated boiling heat transfer coefficients were compared with a new model for refrigerant vaporisation inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 4.7%. The heat transfer and pressure drop measurements are complemented with a IR thermography analysis for a better understanding of the vaporisation process inside a BPHE.