Condensation of Refrigerant R404a in channels of diameter 0.5–3 mm is investigated. The specific focus of this work is to obtain quantitative information on flow mechanisms using image analysis techniques on high speed video. Experiments are conducted on condensing R404a at mass fluxes ranging from 200 to 800 kg m −2 s −1 at vapor qualities of 0.05 to 0.95. The experiments are conducted at the high pressures representative of actual operation of air-conditioning and refrigeration equipment, instead of extrapolating findings for adiabatic air-water mixtures at atmospheric pressure to high pressure refrigerant condensation. Pressure drops are also measured as a function of geometry and operating conditions during the flow visualization experiments. Quantitative image analysis enables detailed computation of void fraction and vapor bubble parameters such as frequency, velocity, diameter and length with a high degree of repeatability and with low uncertainties. The effect of operating conditions on the flow patterns, void fractions, and vapor bubble parameters for saturation temperatures ranging from 30 to 60°C is documented. The resulting void fraction models provide closure for pressure drop and heat transfer models, which has thus far not been possible in the reported works in the literature.

Experimental investigations of a two-phase flow were conducted to study heat transfer and various flow patterns of steam condensation in two different microchannels. Microchannels have a rectangular cross-section with hydraulic diameter of 305μm (depth of 310μm and width of 300μm) and 410.5μm (depth of 312μm and width of 600μm). The length of each microchannel is of 50 mm. The silicon microchannel is covered with a transparent thin Pyrex plate to view different flow patterns. Microthermocouples (K-type, 20μm) were placed in rectangular silicon grooves. Measurements are carried out for different inlet pressures and flow rates of steam while the outlet pressure of the microchannel is kept at atmospheric pressure. Plug/slug flow patterns are observed in the microchannel for different mass fluxes. Local surface temperatures along the microchannel corresponding of each two-phase flow structure are measured and analyzed.

This paper presents the experimental heat transfer coefficient and pressure drop measurements obtained during single phase air heat transfer through four 10 pores per inch (PPI) metal foams with different porosities and different foam core heights, 40 mm and 20 mm respectively. The specimens have been inserted in an open-circuit type wind channel with rectangular cross section. The effect of the foam core height on the heat transfer has been analysed by imposing three constant heat flow rates at the bottom plate of the samples: 250, 325 and 400 W and varying the air mass velocity between 0.005 and 0.025 kg m −2 s −1 at around atmospheric pressure. The temperatures of the base, of the upper plate and of the air, at the inlet and outlet of the test section, have been measured by means of several T-type calibrated thermocouples. The pressure drops during air flow through the samples are recorded by means of a high accuracy differential pressure transducer. The experimental results are presented in terms of heat transfer coefficient, normalised mean wall temperature, pressure gradient, permeability and inertia coefficient. The measured pressure gradients and heat transfer coefficients have been compared against the predictions of two different models recently developed by present authors.

Films are ubiquitous in nature and play an important role in our daily life. The paper focuses on the recent progress that has been achieved in the interfacial thermal fluid phenomena in thin liquid films and rivulets through conducting experiments and theory. Phase shift schlieren technique, fluorescence method and infrared thermography have been used. A spanwise regular structures formation was discovered for films falling down an inclined plate with a built-in local rectangular heater. If the heating is low enough, a stable 2D flow with a bump at the front edge of the heater is observed. For lager heat flux this primary flow becomes unstable, and the instability leads to another steady 3D flow, which looks like a regular structure with a periodically bent leading bump and an array of longitudinal rolls or rivulets descending from it downstream. The heat flux needed for the onset of instability grows almost linearly with the increase of Re number. Strong surface temperature gradients up to 10–15 K/mm, both in the streamwise and spanwise directions have been measured. For a wavy film it was found that heating may increase the wave amplitude because thermocapillary forces are directed from the valley to the crest of the wave. Thin and very thin (less than 10 μm) liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Development of such technology requires significant advances in fundamental research, since the stability of joint flow of locally heated liquid film and gas is a rather complex problem. Experiments with water and FC-72 in flat channels (height 0.2–2 mm) have been conducted. Maps of flow regimes were plotted. It was found that stratified flow exists and stable in the channels with 0.2 mm height and 40 mm width. The critical heat flux for a shear driven film may be up to 10 times higher than that for a falling liquid film, and reaches 400 W/cm 2 in experiments with water at atmospheric pressure. Some experiments have been done during parabolic flight campaigns of the European Space Agency under microgravity conditions. It was found that decreasing of gravity leads to a flow destabilization.

Nucleate boiling of binary mixtures is of particular importance in chemical and process industries. The purpose of the present study is to provide experimental data and prediction method for nucleate boiling heat transfer in two types of anti-freeze solutions, Propylene-glycol (PG)/water and Ethylene-glycol (EG)/water, under atmospheric pressure. The experiments were performed in a saturated condition. The concentrations of solutions are varied from 10 to 40 wt%. It was found that the heat transfer coefficient gradually decreases as increasing the addition of anti-freeze. An addition of small amount of propylene-glycol and ethylene-glycol to water also decreases the CHF value far below that of pure water. It is concluded that the correlation proposed by Fujita for several binary mixtures can well predict the heat transfer coefficients within almost ±5% accuracy for every concentration of mixture solutions.

Heat transfer characteristics from sintered microporous coating surfaces are experimentally investigated for subcooled flow boiling in the horizontal, rectangular small channel with the low aspect ratio. The channel has a hydraulic diameter of 1.98 mm and a heated length of 150 mm. Experiments are conducted at atmospheric pressure with fully-degassed deionized water as the working fluid. The liquid mass flux ranges from 93.6 to 187.1 kg/m 2 s, and the inlet subcooling ranges from 30 to 70 K. The heating surface of the channel is covered with the copper microporous coatings to enhance the nucleate boiling heat transfer. Four coatings sintered with differently sized particles are tested to reveal influences of microporous parameters and identify the optimum structure. A smooth (highly polished) surface is also tested as reference. Although few studies have been performed to quantify the heat transfer enhancement contributed by combining internal porous coatings and small channels, this combination has promising applications in many areas such as air conditioning, chip cooling, refrigeration systems, and many others involving compact heat exchangers.

Two-phase flow on the shell side of a shell and tube heat exchanger is complex. Several studies have produced flow pattern maps that show surprising differences in flow regime boundaries for data sets that contain relatively small variations in fluid and flow properties. Despite this, correlations for void fraction and pressure drop are sufficiently accurate to allow the thermal-fluid design of heat exchangers to be completed. However, these correlations are based on experimental data taken from tube bundles containing tubes with diameters less than 20 mm. This study examines their applicability to tube bundles containing tubes with a diameter of 38 mm. Results for void fraction and pressure drop are presented for air-water flows near atmospheric pressure. The results were obtained for flows through a thin-slice, in-line tube bundle containing 10 rows. The tube bundle contained a central column of tubes with half tubes placed on the shell wall to simulate the presence of other columns. The tubes were 38 mm in diameter and 50 mm long with a pitch to diameter ratio of 1.32. Previous studies have shown that the void fraction in a shell-side, gas-liquid flow becomes constant after only a few rows. Thus, the void fraction was only measured at one location. A single-beam, gamma-ray densitometer was used to measure void fractions near row 7 in the maximum gap between the rows. Corresponding pressure drops were obtained between rows 3 and 10. Data are presented for a mass flux range of 25–688 kg/m 2 s and a gas mass fraction range of 0.0005–0.6. The measurements are shown to compare reasonably well with predictions from correlations available in the open literature. This shows that these methods can be used for tube-bundles containing larger diameter tubes. Some elements of a heat-exchanger design require a more complex analysis. For example, tube vibration calculations require the distribution of void and phase velocity along the tube length. Such analysis can be provided by multiphase computational fluid dynamic (CFD) simulations. CFD approaches to modelling these flows require empirical inputs for the drag coefficient and the force on the fluid by the tubes. These are deduced from the measured data. The wall forces are shown to scale well with increased tube diameter, however, caution is required when selecting the drag coefficients.

The cooling of horizontal wires by natural convection of air is investigated. Experiments are carried out with the use of an apparatus designed for thermal testing of electric cables for aerospace vehicles. Two wire diameters are considered: D = 0.155 mm and 0.593 mm with a length of about 2.45 m. Air pressure is varied from the atmospheric value down to 0.4 mbar, to which corresponds a Knudsen number ranging from 2·10 −4 up to 2 while the Rayleigh number covers ten order of magnitude, from 10 −10 up to 1. On the basis of the value of the Knudsen number, three regimes of heat transfer are identified: free convection in fully continuum fluid regime, the combined free molecule conduction close to the wire surface and free convection in the remaining space and the conductive regime at low pressure, when the rarefied gas is nearly quiescent. An attempt is also made to correlate the heat transfer results in all three regimes.

New experimental data are reported for condensation of ethylene glycol at near atmospheric pressure and low velocity on six three-dimensional pin-fin tubes. Enhancements of the vapour-side, heat-transfer coefficients were found between 3 to 5.5 when compared to a plain tube at the same vapour-side temperature difference. Heat-transfer enhancement was found to be strongly dependent on the active surface area i.e. on the proportion of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapour-side, heat-transfer enhancements were found to be approximately 3 times the corresponding active-area enhancements. The best performing pin-fin tube gave a heat-transfer enhancement of up to 5.5; 17% higher than those obtained from ‘optimised’ two-dimensional fin-tubes reported in the literature and about 24% higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with same fin root diameter, longitudinal fin spacing and thickness and fin height).

An experimental study is undertaken to investigate the flow structure and heat transfer in a stagnation flow Chemical Vapor Deposition (CVD) reactor at atmospheric pressure. It is critical to develop models that predict flow patterns in such a reactor to achieve uniform deposition across the substrate. Free convection can negatively affect the gas flow as cold inlet gas impinges on the heated substrate, leading to vortices and disturbances in the normal flow path. This experimental research will be used to understand the buoyancy-induced and momentum-driven flow structure encountered in an impinging jet CVD reactor. Investigations are conducted for various operating and design parameters. A modified stagnation flow reactor is built where the height between the inlet and substrate is reduced when compared to a prototypical stagnation flow reactor. By operating such a reactor at certain Reynolds and Grashof numbers it is feasible to sustain smooth and vortex free flow at atmospheric pressure. The modified stagnation flow reactor is compared to other stagnation flow geometries with either a varied inlet length or varied heights between the inlet and substrate. Comparisons are made to understand the impact of such geometric changes on the flow structure and the thermal boundary layer. In addition, heat transfer correlations are obtained for the substrate temperature. Overall, the results obtained provide guidelines for curbing the effects of buoyancy and for improving the flow field to obtain greater film uniformity when operating a stagnation flow CVD reactor at atmospheric pressure.

The presence of surfactant additives in water was found to enhance significantly the boiling heat transfer. The objective of the present investigation was to compare the bubble growth in water to that of a surfactant solution with negligible environmental impact. The study was conducted to clarify the effect of the heat flux on the dynamics of bubble nucleation. The bubble growth under condition of pool boiling in water and surfactant solutions was studied using high speed video technique. The bubble generation was studied on a horizontal flat surface; where the natural roughness of the surface was used to produce the bubbles. At heat flux of q= 10 kW/m 2 the life-time and the volume of bubble growth in surfactant solution did not differ significantly from those of water. The time behavior of the contact angle of bubble growing in surfactant solution is qualitatively similar to that of water. At a heat flux of q= 50 kW/m 2 , boiling in surfactant solution, when compared with that of pure water, was observed to be more vigorous. Surfactant promotes activation of nucleation sites; the bubbles appeared in a cluster mode; the life-time of each bubble in the cluster is shorter than that of a single water bubble. The detachment diameter of water bubble increases with increasing heat flux, whereas analysis of bubble growth in surfactant solution reveals the opposite effect: the detachment diameter of the bubble decreases with increasing heat flux. Natural convection boiling of water and surfactants at atmospheric pressure in narrow horizontal annular channels was studied experimentally in the range of Bond numbers Bo = 0.185–1.52. The flow pattern was visualized by high-speed video recording to identify the different regimes of boiling of water and surfactants. The channel length was 24mm and 36mm, the gap size was 0.45, 1.2, 2.2, and 3.7mm. The heat flux was in the range of 20–500 kW/m 2 , the concentration of surfactant solutions was varied from 10 to 600 ppm. For water boiling at Bond numbers Bo<1 the CHF in restricted space is lower than that in unconfined space. This effect increases with increasing the channel length. For water at Bond number Bo = 1.52, boiling can almost be considered as unconfined. Additive of surfactant led to enhancement of heat transfer compared to water boiling in the same gap size, however, this effect decreased with decreasing gap size. For the same gap size, CHF in surfactant solutions was significantly lower than that in water. Hysteresis was observed for boiling in degraded surfactant solutions.

A test rig was constructed to investigate flow boiling in an electrically heated horizontal mini-channel array. The test section is made of copper and consists of twelve parallel mini-channels. The channels are 1 mm deep, 1 mm wide and 250 mm long. The test section is heated from underneath with six cartridge heaters. The channels are covered with a glass plate to allow visual observations of the flow patterns using a high-speed video-camera. The wall temperatures are measured at five positions along the channel axis with two resistance thermometers in a specified distance in heat flow direction. Local heat transfer coefficients are obtained by calculating the local heat flux. The working fluids are deionised water and ethanol. The experiments were performed under near atmospheric pressure (0.94 bar to 1.2 bar absolute). The inlet temperature was kept constant at 20°C. The measurements were taken for three mass fluxes (120; 150; 185 kg/m 2 s) at heat fluxes from 7 to 375 kW/m 2 . Heat transfer coefficients are presented for single phase forced convection, subcooled and saturated flow boiling conditions. The heat transfer coefficient increases slightly with rising heat flux for single phase flow. A strong increase is observed in subcooled flow boiling. At high heat flux the heat transfer coefficient decreases slightly with increasing heat flux. The application of ethanol instead of water leads to an increase of the surface temperature. At the same low heat flux flow boiling heat transfer occurs with ethanol, but in the experiments with water single phase heat transfer is still dominant. It is because of the lower specific heat capacity of ethanol compared to water. There is a slight influence of the mass flux in the investigated parameter range. The pictures of a high-speed video-camera are analysed for the two-phase flow-pattern identification.

Experiments on saturated pool boiling were conducted by using alcohol aqueous solutions of 1-Propanol/Water, 2-Propanol/Water and Water/Ethylene glycol mixtures at atmospheric pressure. The heating surface was a horizontal upward-facing circular flat plate of 40mm in diameter. Despite that only the deterioration of heat transfer coefficient due to the existence of diffusion resistance was reported so far in nucleate boiling of mixtures, enhancement of heat transfer coefficients was observed for 1-Propanol/Water and 2-Propaonl/Water mixtures at extremely low alcohol concentrations. In these concentration ranges, Marangoni force acting on the vapor-liquid interface towards the three-phase interlines was expected to enhance the heat transfer, and this heat transfer enhancement can overcome the heat transfer deterioration due to the mass transfer resistance inherent in mixture boiling. In the intermediate concentration range, however, no heat transfer enhancement but only deterioration was observed, where Marangoni effect had no important role. On the other hand, the measured CHF values were gradually decreased with the increase of alcohol concentration in the moderate concentration range. In the low alcohol concentration range, however, CHF values were clearly decreased for 1-Propanol/Water and 2-Propanol/Water. To confirm the observed heat transfer trends of three different alcohol aqueous solutions, Marangoni effect on the nucleate boiling heat transfer mechanisms was discussed.