Outlet Vapor Quality Research Articles

Forced convection boiling heat transfer inside helically-coiled heat exchanger

For decades, forced convection boiling heat transfer has been considered as one of the most efficient type in the heat transfer mechanism. It has been widely utilized in heat transfer equipment of various industries. Meanwhile, helically-coiled heat exchanger has been commonly utilized in numerous industrial applications. Thus, there is an interest to utilize forced convection boiling heat transfer in helically-coiled heat exchanger. Boiling phenomenon inside helically-coiled tube is more complex as compared to the straight tube due to the secondary flow induced by centrifugal force. This study investigates flow boiling heat transfer performance of water-vapor inside helically-coiled tube by using computational fluid dynamic approach. A Eulerian-Eulerian two-fluid model is used to capture interphase exchange forces and heat transfer between liquid and vapor phase. Wall boiling model is adopted to take into account the boiling condition in the vicinity of the wall. The developed model is then validated against the previously published experimental data. Good agreement for the outlet vapor quality and pressure drop between numerical study and experimental measured value is achieved. The result reveals that the boiling starts at the inner wall of the tube (φ=450°) due to the presence of secondary flow induced by coil curvature. The relationship between HTC and vapor quality along the helically-coiled tube are discussed and evaluated. This study serves as a guideline for future study on forced convection boiling heat transfer in the helically-coiled tube.

Experimental Study of Flow Boiling Heat Transfer of R30 in Microchannels

Two-phase cooling technique including flow boiling heat transfer in microchannel is being widely applied in the mechanical and electronic devices, whereas most of the existing related studies still experiment with common coolants with low boiling points that are not suitable for extreme high-temperature heat dissipation. Thus the flow boiling heat transfer characteristics of R30 have been analyzed through an experiment in parallel microchannels with diameters of 0.5 and 1.0 mm. The experiment was performed under the following conditions: heat flux, ; mass flux, ; outlet vapor quality, 0–0.52. Results showed that the heat transfer coefficient is determined by heat flux and vapor quality in low-vapor-quality region dominated by nucleate boiling, and affected by vapor quality and mass flux in high-vapor-quality region dominated by convective boiling. Intermittent dryout occurs in the nucleate boiling region and is affected by mass flux resulting from the suppression of nucleate boiling. Higher mass flux can relieve the intermittent dryout and postpone the onset of the dryout incipience to higher vapor quality. Comparison with previous correlations showed their unsatisfactory accuracy on present data and implied relatively well heat transfer performance of R30.

Evaluation of Two-Phase Frictional Pressure Drop Correlations Against Experimental Data

AbstractResults are presented here from an experimental investigation on tube side two-phase characteristics that took place in four tested tubes—the 1EHT-1, 1EHT-2, 4LB, and smooth tubes. The equivalent outer diameter of the tube was 9.52 mm and the inner diameter was 8.32 mm. Condensation tests were conducted using refrigerant R410A at a saturation temperature of 318 K, over a mass flow range of 150–450 kg m−2 s−1, with inlet and outlet vapor qualities of 0.8 and 0.2, respectively. Evaporation tests were performed at a saturation temperature of 279 K, over a mass flow range of 150–380 kg m−2 s−1, with inlet and outlet vapor qualities of 0.2 and 0.8, respectively. Pressure drop data of the four tested tubes were collected to evaluate five identified prediction correlations based on the separated flow model and the homogeneous flow model. The separated flow approaches presented predictions with average MAEs of 24.9% and 16.4% for condensation and evaporation data, respectively, while the average MAEs of the homogeneous flow model were 31.6% and 43.4%, respectively. Almost all the identified correlations underestimated the frictional pressure drop of the 4LB tube with MAEs exceeding 30%. An earlier transition of different flow patterns was expected to occur in the EHT tubes while developing a new diabatic flow pattern map is needed for the 4LB tube. A new correlation was presented based on the two-phase multiplier Φ and the Martinelli parameter Xtt, which exhibited excellent predictive results for the experimental data.

Flow boiling in horizontal annuli outside horizontal smooth, herringbone and three-dimensional enhanced tubes

Heat transfer and pressure drop measurements of R410A saturated flow boiling in horizontal annuli were performed at a saturation temperature of 6 °C over the mass flux range of 8–107 kg/(m2 s). The inner tubes with the same inside diameter of 12.70 mm contain a smooth tube, a herringbone micro-fin tube and a 1EHT tube (a dimpled tube enhanced by petal-shaped background patterns). In this study, the test section is a typical double-tube type heat exchanger and the emphasis is focused on the effects of mass flux, vapor quality, annular width and surface structures on the annulus-side heat transfer. Wilson plot method was adopted to ensure the correction factor of water-side coefficient of 1EHT tube. For flow boiling tests in annuli with an annular gap of 6.35 mm, both the evaporating coefficient and pressure drop increase as the mass velocity increases. Better thermal performance was seen at a outlet vapor quality of 0.6. In this case, the heat transfer rate ratio of enhanced annulus to smooth annulus is 1.7–2.5 for the herringbone micro-fin tube and 1.4–1.7 for the 1EHT tube, respectively. When the annular spacing decreases, heat transfer deterioration will worsen accompanied with a sudden drop in boiling heat-transfer coefficient. Besides, a comparison of flow boiling characteristics of annulus-side and tube-side heat transfer was made. The annulus-side heat transfer coefficient is much larger than that of tube-side case.

Two-phase heat transfer in horizontal dimpled/protruded surface tubes with petal-shaped background patterns

Experimental investigations were carried out to measure the average heat transfer coefficient and frictional pressure loss of R410A during condensation and evaporation in five newly-developed dimpled tubes and two smooth tubes. These dimpled tubes were called EHT series tubes enhanced by dimple or protrusion arrays and staggered petal-shaped background patterns on their inner surface. Tests were conducted at three condensation temperatures and two evaporation temperatures; over a mass flux range of 60–450 kg/(m2·s); with an inlet vapor quality of 0.8/0.2 and outlet vapor quality of 0.2/0.8. Among the copper test tubes having the same outer diameter, the 1EHTa tube (dimpled surface tube) showed the highest condensing coefficient and pressure drop under the same test conditions. On the contrary, the superior evaporation heat transfer performance was given by the 1EHTb tube (protruded surface tube). In addition, the positive effects of small tube diameter and low saturation temperature on thermal performance have been confirmed. It was also observed that measured heat transfer coefficient and pressure drop of the copper 1EHTa tube were higher in comparison with the stainless steel one. The main reason could be the low thermal conductivity of stainless steel material. A comparison of heat transfer enhancement in these enhanced tubes has been done. Results indicated that both the copper 1EHTa and 1EHTb tubes provided the satisfactory surface thermal efficiency.

A diamond made microchannel heat sink for high-density heat flux dissipation

Flow boiling in microchannels is a promising technique for cooling high power-density electronic devices. In this study, a microchannel heat sink using ammonia as working fluid is developed, and its cooling efficiency is experimentally investigated under nonuniform high-density heat flux, which well simulates a practical heat dissipation scenario for microthermal systems. Diamond with high thermal conductivity of 1500 W/m·K is selected as the microchannel heat sink material. A total of 37 parallel triangular microchannels with aspect ratio of 5, channel length of 45 mm and hydraulic diameter of 280 μm are uniformly engineered on the diamond film by laser ablation processing. The significance of diamond substrate as a heat spreader to minimize the nonuniformity of heat flux imposed by a central hotspot is verified. The influences of heat flux and mass flux on the cooling efficiency are experimentally investigated. An optimal range of outlet vapor quality from 0.10 to 0.13 is found, within which the minimum heat source temperature can be achieved. Notably, the microchannel heat sink is capable of managing a central hotspot with heat flux of 267 W/cm2 while maintaining the heat source temperature at 53.3 °C for a mass flux of 320 kg/m2s.

Experimental study on convective boiling of micro-pin-finned channels with parallel arrangement fins for FC-72 dielectric fluid

The boiling convective heat transfer of dielectric fluid of FC-72 in a microchannel with etched micro fins is experimentally studied. The microchannel test surface is a square of 10 mm size with a height of 100 μm. The surface of the microchannel is covered with parallel-nucleated fins of a cubic size of 100 μm. The fins are nucleated with the central pore size of 60 μm and an opening of 45 μm. The fins are arranged in the equal space of 400 μm. There is a planar electrical element below the microchannel, which produces an adjustable surface heat flux below the channel. The dielectric liquid enters the test microchannel and flows over the test surface and the fins. In the experiments, the mass velocity (94–275 kg/m2 s) and heat flux (0–6 W/cm2) are tested with a saturation temperature of either 35 or 50 °C. The heat transfer, the outlet vapor quality, and the pressure drop across the microchannel are measured and reported. The boiling behavior of FC-72 over the etched fins is also investigated using photos. The results show that the liquid first go through some superheat states and then the boiling phase change starts. When the boiling heat transfer occurs, a significant pressure-drop can be observed across the microchannel, but the surface temperature difference would also drop significantly. In the case of very low mass velocity of 94 kg/m2 s, the minimum superheat temperature-difference of 4.7 °C is observed.

Flow Boiling Heat Transfer Characteristics in Horizontal, Three-Dimensional Enhanced Tubes

An experimental investigation was conducted to explore the flow boiling heat transfer characteristics of refrigerants R134A and R410A inside a smooth tube, as well as inside two newly developed surface-enhanced tubes. The internal surface structures of the two enhanced tubes are comprised of protrusions/dimples and petal-shaped bumps/cavities. The equivalent inner diameter of all tested tubes is 11.5 mm, and the tube length is 2 m. The experimental test conditions included saturation temperatures of 6 °C and 10 °C; mass velocities ranging from 70 to 200 kg/(m2s); and heat fluxes ranging from 10 to 35 kW/m2, with inlet and outlet vapor quality of 0.2 and 0.8. It was observed that the enhanced tubes exhibit excellent flow boiling heat transfer performance. This can be attributed to the complex surface patterns of dimples and petal arrays that increase the active heat transfer area; in addition, more nucleation sites are produced, and there is also an increased interfacial turbulence. Results showed that the boiling heat transfer coefficient of the enhanced surface tubes was 1.15–1.66 times that of the smooth tubing. Also, effects of the flow pattern and saturated temperature are discussed. Finally, a comparison of several existing flow boiling heat transfer models using the data from the current study is presented.

Three-Dimensional Numerical Study of R134a Flow Boiling Characteristics in a Circular Tube under Overload

Three-dimensional numerical study of R134a flow boiling under overload was conducted based on the VOF model. A series of simulations were implemented with a 1.002 mm circular tube under six gravity levels ranging from 1 to 15 g and three directions (θ) ranging from 0° to 180° with mass fluxes of 200, 350, and 500 kg/m2 s, outlet vapor qualities of 0.041, 0.082, and 0.164, and saturation pressure of 0.71MPa. The gravity conditions were introduced into the momentum equation during the simulations. Comparisons among flow boiling characteristics under different gravity levels and directions are made, and significant effects of overload are found. With the increasing overload, the vapor phase distributions of flow boiling gradually turn into forms similar to ellipsoids/spheres, stratified flow, and elongated slugs, at θ = 0°, 90°, and 180°, respectively. Accordingly, the heat transfer coefficient keeps almost stable at θ = 0° and 180°, but decreases obviously at θ = 90°; the frictional pressure drop decreases at θ = 0°, decreases slightly at θ = 90°, but increases at θ = 180°. Making θ away from 90° is a potential method to avoid the flow boiling heat transfer deterioration under overload.

Evaporation heat transfer characteristics of R410A inside horizontal three-dimensional enhanced tubes

An experimental investigation was conducted for flow boiling inside three horizontal stainless-steel tubes with a series of newly-developed three-dimensional enhanced surfaces. Both the inner and outer surfaces of the 1EHT tubes were composed of dimples in different sizes and depths, and with an array of rectangular cavities for the 3EHT tube. These enhanced tubes have the same outer and inner diameters of 9.52 mm and of 8.32 mm, respectively. The evaporation experiments were performed using R410A as the working fluid at a constant saturation temperature of 279 K. The mass fluxes varied from 160 to 390 kg/m2s, with inlet and outlet vapor qualities of 0.2 and 0.8, respectively. As the mass velocity increased, the heat transfer coefficient and pressure drop penalty increased accordingly. Experimental results showed that the pressure drop of the 1EHT-1 tube is larger than those of the 1EHT-2 and 3EHT tubes, while the 1EHT-2 tube has the highest evaporation heat transfer coefficient.

Condensation and evaporation characteristics of flows inside vipertex 1eht and 4eht small diameter enhanced heat transfer tubes

Results are presented here from an experimental investigation on tubeside condensation and evaporation heat transfer that took place in three Vipertex stainless steel enhanced heat transfer tubes (1EHT-2, 2EHT-2 and 4EHT). Equivalent outer diameter of the tube was 9.52 mm (0.375 in) and the inner diameter was 8.32 mm (0.3276 in). The test apparatus included a horizontal, straight test section with an active length heated by water circulated in a surrounding annulus. Constant heat flux was maintained and refrigerant quality varied. Condensation experimental runs were performed using R410A as the working fluid; over the inlet quality range of 0.2– 0.8; for mass flux values that ranged from 150 to 460 kg/(m2 s). In a comparison of condensation heat transfer performance, the enhanced 1EHT-1 tube has best heat performance followed by the enhanced 4EHT tube and finally the 1EHT-2 tube. The highest pressure drop increase was seen in the 1EHT-1 tube followed by the 4EHT tube and the 1EHT-2 tube. The evaporation experiments were performed using R410A at a constant saturation temperature of 279 K; for a mass flux that ranged from 160 to 390 kg/(m2 s). Inlet and outlet vapor qualities were fixed at 0.2 and 0.8, respectively. As the mass velocity increased, the heat transfer coefficient and pressure drop penalty increase accordingly. Experimental results show a slightly larger pressure drop in the 1EHT-1 tube. The pressure drop in the three EHT tubes could be attributed to the dimples and protrusions in the surface structure, which produce an increased density of nucleation sites. In addition, it was found that the evaporation pressure drop increases with the increasing depth of the dimples. Condensation and evaporation performance is mainly due to the increase in the heat transfer surface area and the increase of interfacial turbulence; this produces flow separation, secondary flows and a higher heat flux from the wall to the working fluid. Enhanced heat transfer tubes are important options to be considered in the design of high efficiency systems. A wide variety of industrial processes involve the transfer of heat energy during phase change and many of those processes employ old technology. These processes are ideal candidates for a redesign that could achieve improved process performance. Vipertex enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products.

Investigation on flow condensation of refrigerant in annulus of smooth and enhanced tube-in-tube heat exchanger

An experimental investigation of R22 and R410A condensation outside a horizontal smooth tube, a herringbone tube and a newly developed 3D enhanced heat transfer (1EHT) dimple tube has been conducted. The herringbone tube has a fin root diameter of 11.43 mm, a helical angle of 21.3 °, 48 fins with a fin height of 0.262 mm and an apex angle of 36 °; the 1EHT tube has an inner diameter of 11.5 mm with a dimple enhancement; while the smooth tube has an inner diameter of 11.43 mm; and all the tubes have an outer diameter of 12.7 mm. Experiments were performed for a constant saturation temperature of 45°C; with a constant inlet vapor quality of 0.8 and a constant outlet vapor quality of 0.1; for a mass flux ranging from 5 kg/(m2 s) to 250 kg/(m2 s). In addition, annular side condensation experiments were performed using an outer shell tube with outer diameters of 17 mm and 25.4 mm. Heat transfer performance varied with mass flux. At a low mass flux the enhanced dimple tube had the smallest heat transfer coefficient; while at higher values of mass flux, the smooth tube had the smallest heat transfer coefficient. Finally, the effect of average vapor quality on the heat transfer coefficient was also investigated. Characteristic analysis was performed in order to account for the various phenomena found in this series of experiments. Annular side heat transfer performance combined with pressure drop measurements reveal that the herringbone tube generally had a better heat transfer performance than the other tubes, and can be a good choice for use in annular side condensation applications.

Condensation heat transfer and pressure drop characteristics of R-134a in horizontal smooth tubes and enhanced tubes fabricated by selective laser melting

This paper reports a study of condensation heat transfer and pressure drop of R134a inside four enhanced tubes and one plain tube fabricated by Selective Laser Melting (SLM). The results are compared to a plain commercial aluminum tube. The enhanced tubes consist of a tube with a metallic foam structure, a tube with eight short circumferential pin fins, a tube with five long circumferential pin fins and a tube with five twisted pin fins. The experiments were conducted at mass fluxes from 50 to 150 kg/m2 s. Throughout the experiments, the inlet and outlet vapor qualities were maintained at 0.9 and 0.3, respectively. Two saturation pressures of 13.4 bar and 11.6 bar were investigated. The effects of fin height, refrigerant flow direction and mass flux on the heat transfer coefficient and pressure drop were studied. Our results show that for the wavy flow pattern, the saturation pressure, vapor quality, mass flux, refrigerant flow direction and fin structure have significant effects on the condensation heat transfer coefficient and pressure drop. At higher saturation pressures, the head impact on the fins with shorter fin height has a higher heat transfer coefficient than the back impact. For the longer and twisted fins, a reversed trend was observed. With an increase in the mass flux or a decrease in the saturation pressure, the difference in heat transfer coefficients between the head and back impact for the same tube structure reduces. The heat transfer coefficients of the metallic foam tubes are higher than that of the plain SLM tube with a large penalty of pressure drop. The eight-fin tubes yield higher efficiency indices in terms of heat transfer over pressure drop when compared to the other tubes.

Condensation heat transfer correlation for water-ethanol vapor mixture flowing through a plate heat exchanger

Condensation heat transfer coefficient (HTC) as a function of outlet vapor quality was investigated using water-ethanol vapor mixture of different ethanol vapor concentrations (0%, 1%, 2%, 5%, 10%, 20%) under three different system pressures (31 kPa, 47 kPa, 83 kPa). A heat transfer coefficient was developed by applying multiple linear regression method to experimental data, taking into account the dimensionless numbers which represents the Marangoni condensation effects, such as Re, Pr, Ja, Ma and Sh. The developed correlation can predict the condensation performance within a deviation range from −22% to 32%. Taking PHE’s characteristic into consideration and bringing in Ma number and Sh number, a new correlation was developed, which showed a much more accurate prediction, within a deviation from −3.2% to 7.9%.

Experimental Study on Flow Boiling of Deionized Water in a Horizontal Long Small Channel

In this paper, an experimental investigation on the flow boiling heat transfer in a horizontal long mini-channel was carried out. The mini-channel was with 2 mm wide and 1 mm deep and 900 mm long. The material of the mini-channel was stainless. The working fluid was deionized water. The experiments were conducted with the conditions of inlet pressure in the range of 0.2~0.5 MPa, mass flux in the range of 196.57-548.96 kg/m2s, and the outlet vapor quality in the range of 0.2 to 1. The heat flux was in the range of 292.86 kW/m2 to 788.48 kW/m2, respectively. The influences of mass flux and heat flux were studied. At a certain mass flow rate, the local heat transfer coefficient increased with the increase of the heat flux. If dry-out occurred in the mini-channel, the heat transfer coefficient decreased. At the same heat flux, the local heat transfer coefficient would depend on the mass flux. It would increase with the mass flux in a certain range, and then decrease if the mass flux was beyond this range. Experimental data were compared with the results of previous studies. Flow visualization and measurements were conducted to identify flow regime transitions. Results showed that there were eight different kinds of flow patterns occurring during the flow boiling. It was found that flow pattern had a significant effect on heat transfer.

Correlation of Evaporation Heat Transfer Inside 8.8 mm and 7.14 mm Horizontal Round Micro-Fin Tubes

ABSTRACTExperimental investigations of evaporation heat transfer of R22 and R410A inside a horizontal micro-fin copper tube have been conducted and are reported here. Six micro-fin tubes with inner diameter of 7.14 mm and three micro-fin tubes with inner diameter of 8.8 mm but with different geometric parameters, such as the apex angle, the helical angle, fin height, fin pitch, and starts were tested. The evaporation experiments were taken at a constant temperature of 6°C. Moreover, working conditions of the experiments varied with the mass flux ranging from 100 kg/(m2.s) to 400 kg/(m2.s). For the evaporation experiments of Tube 1 – Tube 6 with R22, the inlet and outlet vapor quality is set as 0.1 and 0.9, respectively. For the evaporation experiments of Tube 7 – Tube 9 with R410A, the inlet and outlet vapor quality is set as 0.2 and 0.9, respectively. The heat transfer coefficients and the changing trend of the heat transfer coefficients vary among these tubes. The influence of each geometric parameter on the heat transfer performance of the micro-fin tube has been analyzed and is reported. Besides, correlations of evaporation heat transfer inside 8.8 mm or less horizontal round micro-fin tubes were developed.

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.

The influence of cavitation on the flow characteristics of liquid nitrogen through spray nozzles: A CFD study

Spray cooling with cryogen could achieve lower temperature level than refrigerant spray. The internal flow conditions within spray nozzles have crucial impacts on the mass flow rate, particle size, spray angle and spray penetration, thereby influencing the cooling performance. In this paper, CFD simulations based on mixture model are performed to study the cavitating flow of liquid nitrogen in spray nozzles. The cavitation model is verified using the experimental results of liquid nitrogen flow over hydrofoil. The numerical models of spray nozzle are validated against the experimental data of the mass flow rate of liquid nitrogen flow through different types of nozzles including the pressure swirl nozzle and the simple convergent nozzle. The numerical studies are performed under a wide range of pressure difference and inflow temperature, and the vapor volume fraction distribution, outlet vapor quality, mass flow rate and discharge coefficient are obtained. The results show that the outlet diameter, the pressure difference, and the inflow temperature significantly influence the mass flow rate of spray nozzles. The increase of the inflow temperature leads to higher saturation pressure, higher cavitation intensity, and more vapor at nozzle outlet, which can significantly reduce mass flow rate. While the discharge coefficient is mainly determined by the inflow temperature and has little dependence on the pressure difference and outlet diameter. Based on the numerical results, correlations of discharge coefficient are proposed for pressure swirl nozzle and simple convergent nozzles, respectively, and the deviation is less than 20% for 93% of data.

Shell-Side Flow Condensation of R410A on Horizontal Tubes at Low-Mass Fluxes

An investigation of refrigerant R410A condensation on a shell and tube heat exchanger simulation is conducted. Tests are on the outside of a horizontal smooth tube, a herringbone tube, and a newly developed three-dimensional-enhanced tube, called the enhanced tube (EHT) tube, all of the same outer diameter. Experiments were conducted at a constant saturation temperature of 45 °C, a constant inlet vapor quality of 0.8, a constant outlet vapor quality of 0.1, and mass fluxes ranging from 5 kg/(m2 s) to 50 kg/(m2 s). At low-mass velocities, the smooth tube shows superior performance over the herringbone tube and the EHT tube. The cause might lie in surface tension effects that result in liquid inundation at the lower portion of the tube, thickening the film on the tube and deteriorating the heat transfer performance. Analyses were conducted to find a suitable correlation of the experimental data.

Flow Boiling Heat Transfer and Dryout Characteristics of R600a in a Vertical Minichannel

Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60 mm inside diameter and 245 mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32°C, with five mass fluxes in the range 50–350 kg/m2-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.