Flow boiling heat transfer study of R-134a/R-290/R-600a mixture in 9.52 and 12.7 mm smooth horizontal tubes: Experimental investigation

A detailed experimental investigation is carried out to study the flow boiling heat transfer behavior of R-134a/R-290/R-600a (91%/4.068%/4.932% by mass) refrigerant mixture in smooth horizontal tubes of diameter 9.52 and 12.7 mm. The heat transfer coefficients of the mixture are experimentally measured under varied heat flux conditions for stratified flow patterns using a coaxial counter-current heat exchanger test section. The tests are conducted for refrigerant inlet temperatures between ?9 and 5 °C and mass flow rates ranging from 3 to 5 g s^?1. Kattan–Thome–Favrat maps are used to confirm the flow patterns for the tested conditions. The magnitude of the heat transfer coefficient with respect to flow patterns and different mechanisms of boiling are discussed. The heat transfer coefficient of the refrigerant mixture is also compared with that of R-134a for selected working conditions. The significance of nucleate boiling in the overall heat transfer process under these testing conditions is highlighted.     

Low-GWP refrigerants flow boiling heat transfer in a 5 PPI copper foam

This paper reports an experimental investigation of the heat transfer performance of the new low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling heat transfer inside a horizontal high porosity copper foam with 5 Pores Per Inch (PPI). Metal foams are a class of cellular structured materials consisting of a stochastic distribution of interconnected pores; these materials have been proposed as effective solutions for heat transfer enhancement during both single and two-phase heat transfer. R1234yf and R1234ze(E) refrigerants are appealing alternatives of the more traditional R134a by virtue of their negligible values of GWP and normal boiling temperatures close to that of R134a, which make them suitable solution in several different applications, such as: refrigeration and air conditioning and electronic thermal management. This work compares the two-phase heat transfer behaviour of these new HFO refrigerants, studying the boiling process inside a porous medium and permitting to understand their effective heat transfer capabilities. The experimental measurements were carried out by imposing three different heat fluxes: 50, 75, and 100 kW m⁻², at a constant saturation temperature of 30 °C; the refrigerant mass velocity was varied between 50 and 200 kg m⁻² s⁻¹, whilst the mean vapour quality varied from 0.2 to 0.95. The two-phase heat transfer and pressure drop performance of the two new HFO refrigerants is compared against that of the more traditional R134a.     

Flow boiling heat transfer of R134a,R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m²s, heat fluxes from 2.3 kW/m² to 250 kW/m² at T_sat = 31 °C and ΔT_sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.     

Saturated nucleate boiling to binary and ternary mixtures on horizontal cylinder

In this investigation, a large number of experiments have been performed to determine saturated nucleate pool boiling heat transfer coefficients of MEA/water and DEA/water binary mixtures and that of water/MEA/DEA ternary mixtures. These heat transfer coefficients have been measured at atmospheric pressure and over a wide range of heat fluxes and solution concentrations. The heat flux has been varied in 14 different levels from 7 to about 230 kW/m² and amines concentration has been changed in 10 different levels from zero to 84 wt%. Results show that strong reduction of heat transfer coefficient occurs as a result of mass transfer interference in this phenomenon. Furthermore, in this study, all the correlations proposed during the last years for the prediction of nucleate boiling heat transfer coefficient of mixtures have been categorized in three groups. Some experimental results have been compared with the most accurate representatives of these three groups and the corresponding RMS errors have been calculated. Also, impacts of important existing parameters in these correlations like ideal heat transfer coefficient (h_id.) on the prediction have been discussed.     

Experimental study on bubble departure characteristics in subcooled nucleate pool boiling

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300 mm × 135 mm × 250 mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (ΔT_sub = 5−20 K), superheat (ΔT_sat = 1−10 K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value.     

Numerical simulation of flow boiling for organic fluid with high saturation temperature in vertical porous coated tube

A semi-analytical model is developed for the prediction of flow boiling heat transfer inside vertical porous coated tubes. The model assumes that the forced convection and nucleate boiling coexist together in the annular flow regime. Conservations of mass, momentum, and energy are used to solve for the liquid film thickness and temperature. The heat flux due to nucleate boiling consists of those inside and outside micro-tunnels. To close the equations, a detailed analysis of various forces acting on the bubble is presented to predict its mean departure diameter. The active nucleation site density of porous layer is determined from the pool boiling correlation by introducing suppression factor. The flow boiling heat transfer coefficients of organic fluid (cumene) with high saturation temperature in a vertical flame-spraying porous coated tube are studied numerically. It is shown that the present model can predict most of the experimental values within ±20%. The numerical results also indicate that the nucleate boiling contribution to the overall heat transfer coefficient decreases from 50% to 15% with vapor quality increasing from 0.1 to 0.5.     

Nucleate boiling in graphene oxide colloids: Morphological change and critical heat flux enhancement

We report an experimental investigation into the boiling heat transfer characteristics of water based graphene oxide (GO) colloidal suspensions on a silicon dioxide plate surface. Pool boiling experiments were conducted with GO colloidal suspensions at three concentrations: 1, 5, and 10 mg/L. The characteristics of the GO colloidal suspensions were compared to those of reduced GO (RGO) colloidal suspensions examined in our previous studies, in which we investigated the critical heat flux (CHF) enhancement mechanism in graphene colloids. Even though GO and RGO share almost identical geometrical and chemical characteristics, the GO and RGO flakes formed a two-dimensional (2D) laminate film and a three-dimensional (3D) porous network on a 2D film by nucleate boiling, respectively. Both species showed dramatic CHF enhancement, but not surface wettability enhancement. The 2D structure formed by nucleate boiling exhibited a delayed CHF phenomenon. We found that the GO flakes were well-aligned in the 2D film, such that the film acted as a heat spreader, with superior thermal conductivity. The CHF enhancement data were compared with thermal activity analysis results. Boiling hydrodynamics was examined under high-speed camera visualization.     

Enhanced heat transfer in confined pool boiling

We report the results of an experimental investigation of the heat transfer during nucleate boiling on a spatially confined boiling surface. The heat flux as a function of the boiling surface temperature was measured in pool boiling pots with diameters ranging from 15 mm down to 4.5 mm. It was found that a reduction of the pool diameter leads to an enhancement of the nucleate boiling heat flux for most of the boiling curve. Our experimental results indicate that this enhancement is not affected by the depth of the boiling pot, the material of the bounding wall, or the diameter of the inlet water supply. High-speed camera imaging shows that the heat transfer enhancement for the spatially confined pool boiling occurs in conjunction with a stable circulating flow, which is in contrast to the chaotic and mainly upward motion for boiling in larger pool diameters. An explanation for the enhancement of the heat transfer and the associated change in flow pattern is found in the singularisation of the nucleate boiling process.     

Flow boiling of halogenated refrigerants at high saturation temperature in a horizontal smooth tube

Several correlations are available in the open literature for computing the heat transfer coefficient during flow boiling inside plain channels. With respect to halogenated refrigerants, these correlations are usually compared to data taken in a limited range of evaporation temperature and reduced pressure. More recently, the adoption of new refrigerants, such as high pressure HFCs and carbon dioxide, requires to largely extend the pressure range of application of such correlations. Besides, the design of evaporators for some heat pumping applications, where temperatures are set at higher values as compared to usual evaporating temperatures in air-conditioning equipment, requires proper validation of the computing methods.The present paper aims at comparing four well-known predicting models to a new database collected during flow boiling of HCFC (R22) and HFC refrigerants (R134a, R125 and R410A) in a horizontal 8 mm internal diameter tube. This database is characterized by saturation temperature ranging between 25 °C and 45 °C, reduced pressure spanning between 0.19 and 0.53. Mass velocity ranges between 200 and 600 kg m^?2 s^?1 and heat flux between 9 and 53 kW m^?2.Evaporating heat transfer coefficients of halogenated refrigerants at such high temperatures have not been reported in the open literature so far. The discussion of the results will enlighten some similarities with experimental trends presented in the literature for evaporating carbon dioxide.Two models tested here show good prediction capabilities of the present experimental data, but not for all the data sets in the same way. For the purpose of practical use, a simple modification of the correlation by Gungor and Winterton [1] is proposed, showing that this is able to catch the experimental trends of the present database with good agreement.     

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m² s, heat flux 120–270 kW/m², vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.     

Experimental pool boiling investigations of FC-72 on silicon with artificial cavities and integrated temperature microsensors

In this experimental study, fluorinert FC-72 is boiled on a silicon chip with artificial cavities and integrated microsensors. The horizontal silicon chip with dimensions of 39.5 × 19 × 0.38 mm is completely immersed in FC-72. The integrated nickel–titanium temperature microsensors on the back of the chip are calibrated individually and exhibit a near-linear increase of electrical resistance with temperature. The applied heat fluxes and the resulting wall superheat at the boiling surface are varied by means of an integral thin-film resistance heater (95% Al, 4% Cu and 1% Si), also on the back of the silicon chip. Artificial cylindrical cavities with a mouth diameter of 10 μm and depths of 40, 80 or 100 μm situated above the microthermometers serve as artificial nucleation sites, due to trapped vapour. Bubble growth rates, frequencies, departure diameters of bubbles and waiting times between bubbles from an isolated cavity for different wall superheats and pressures were obtained by analysing high-speed video images and the simultaneously measured temperature below the artificial cavity.     

Roughness and surface material effects on nucleate boiling heat transfer from cylindrical surfaces to refrigerants R-134a and R-123

This paper presents results of an experimental investigation carried out to determine the effects of the surface roughness of different materials on nucleate boiling heat transfer of refrigerants R-134a and R-123. Experiments have been performed over cylindrical surfaces of copper, brass and stainless steel. Surfaces have been treated by different methods in order to obtain an average roughness, Ra, varying from 0.03 μm to 10.5 μm. Boiling curves at different reduced pressures have been raised as part of the investigation. The obtained results have shown significant effects of the surface material, with brass being the best performing and stainless steel the worst. Polished surfaces seem to present slightly better performance than the sand paper roughened. Boiling on very rough surfaces presents a peculiar behavior characterized by good thermal performance at low heat fluxes, the performance deteriorating at high heat fluxes with respect to smoother surfaces.     

Spatially resolved wall temperature measurements during flow boiling in microchannels

Spatial and temporal variations of channel wall temperature during flow boiling microchannel flows using infrared thermography are presented and analyzed. In particular, the top channel wall temperature in a branching microchannel silicon heat sink is measured non-intrusively. Using this technique, time-averaged temperature measurements, with a spatial resolution of 10 μm, are presented over an 18 mm × 18 mm area of the heat sink. Also presented, within a specific sub-region of the heat sink, are intensity maps that are recorded at a rate of 120 frames per second. Time series data at selected point locations in this sub-region are analyzed for their frequency content, and dominant temperature fluctuations are extracted using proper orthogonal decomposition.Results at low-vapor-quality boiling condition indicate that temperatures can be determined from recorded radiation intensities with a temperature uncertainty varying from 0.9 °C at 25 °C to 1.0 °C at 125 °C. The time series data indicate periodic wall temperature fluctuations of approximately 2 °C that are attributed to the passage of vapor slugs. A dominant band of frequencies around 2–4 Hz is suggested by the frequency analysis. Proper orthogonal decomposition results indicate that first six orthogonal modes account for approximately 90% of the variance in temperature. The first mode reconstruction accounts for temporal variations in the dataset in the sub-region analyzed; however the magnitude of fluctuations and spatial variations in temperature are not accurately captured. A reconstruction using the first 25 modes is considered sufficient to capture both the temporal and spatial variations in the data.     

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 (500–1250 kg/m²/s), heat fluxes (100–1600 kW/m²), inlet subcoolings (5, 10, 15 K), 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.     

The Effects of duct height on heat transfer enhancement of a co-rotating type rectangular finned surface in duct

Experiments were performed to investigate the effect of duct height on heat transfer enhancement of a surface affixed with arrays (7 × 7) of short rectangular plate fins of a co-rotating type pattern in the duct. An infrared imaging system is used to measure detailed distributions of the heat transfer at the endwall along with the fin base. An infrared camera of TVS 8000 with 160 × 120 point In–Sb sensor was used to measure the temperature distributions in order to calculate the local heat transfer coefficients of the representative fin regions. Pressure drop and heat transfer experiments were performed for a co-rotating fin pattern varying the duct height from 20?50 mm. The friction factor calculated from the pressure drop shows that comparatively larger friction occurs for the smaller duct cases and the friction factor slowly decreases with increasing Reynolds number. The effect of duct height on the area-averaged heat transfer results show that heat transfer initially increases with duct height and then finally decreases with increasing the duct height. Detailed heat transfer analysis and iso-heat transfer coefficient contour gives a clear picture of heat transfer characteristics of the overall surface. The relative performance graph indicates that a 25 mm duct is the optimum duct height for the highest thermal performance. In addition, a significant thermal enhancement, 2.8?3.8 times the smooth surface, can be achieved at lower Reynolds number with a co-rotating fin pattern in the duct.     

Soret and Dufour effect on double diffusion mixed convection from a vertical surface in a porous medium saturated with a non-Newtonian fluid

A non-similar boundary layer analysis is presented to study the flow, heat and mass transfer characteristics of non-Darcian mixed convection of a non-Newtonian fluid from a vertical isothermal plate embedded in a homogeneous porous medium with the effect of Soret and Dufour and in the presence of either surface injection or suction. The value of the mixed-convection parameter lies between 0 and 1. In addition, the power-law model is used for non-Newtonian fluids with exponent n < 1 for pseudoplastics n = 1 for Newtonian fluids and n > 1 for dilatant fluids. Furthermore, the coordinates and dependent variables are transformed to yield computationally efficient numerical solutions that are valid over the entire range of mixed convection, from the pure forced-convection limit to the pure free-convection limit, and the whole domain of non-Newtonian fluids, from pseudoplastics to dilatant fluids. The numerical solution of the problem is derived using a Runge–Kutta integration scheme with Newton–Raphson shooting technique. Distributions for velocity, temperature and concentration, as well as for the rate of wall heat and mass transfer, have been obtained and discussed for various physical parametric values.     

Experimental investigation of micro-scale temperature transients in sub-cooled flow boiling on a horizontal heater

Surface temperature fluctuations that occur locally underneath departing bubbles in pool boiling are shown to result in local heat transfer coefficients ranging from 1 to 10 kW/cm². These estimates were reported in the literature involved both numerical and experimental approaches. Significantly higher heat fluxes are associated with flow boiling than pool boiling under similar conditions of wall superheat and liquid subcooling (e.g. at boiling inception and at critical heat flux). These enhancements are primarily caused by the convective transport, acceleration/distortion of the bubble departure process as well as the resultant potential enhancement of the local surface temperature fluctuations.In this study we measure the surface temperature fluctuations using temperature micro/nano-sensors fabricated on a silicon wafer during flow boiling on the silicon wafer which is heated from below. The silicon wafer is clamped on a constant heat flux type calorimeter consisting of a vertical copper cylinder with embedded cartridge heaters and K-type thermocouples. Micro/nano-thermocouples (thin film thermocouples or “TFT”) are fabricated on the surface of the silicon wafer. High speed data acquisition apparatus is used to record temperature data from the TFT at 1 kHz. A fluorinert was used as the test fluid (PF-5060, manufacturer: 3M Co.). The calorimeter and surface temperature measurement apparatus is housed in a test section with glass walls for visual observation. The liquid is pumped from a constant temperature bath to maintain a fixed subcooling during the experiments under steady state conditions. The transient temperature data from the FFT array during flow boiling on the silicon wafer is analyzed using fast Fourier transform (FFT). The FFT data is analyzed as a function of the wall heat flux and wall superheat. The number of temperature peaks in the FFT data is observed to increase with increase in wall heat flux and the peaks are found to cover a wider spectrum with peaks at higher frequencies with enhancement of heat flux. The surface temperature fluctuations, especially at small length and time scales, are perturbed potentially by the coupled hydrodynamic and thermal transport processes, resulting in enhanced local and global heat flux values. Boiling incipience condition and the flow boiling data are compared with correlations reported in the literature.     

Local jet impingement boiling heat transfer with R113

An experimental study was performed to characterize the boiling heat transfer of impinging circular submerged jets on simulated microelectronic chips with a nominal area of 5 mm × 5 mm. The heat transfer modes included natural convection, partially developed nucleate boiling, fully developed nucleate boiling and critical heat flux. The study included the effects of jet parameters and fluid subcooling on the nucleate boiling. The results showed that the nucleate boiling data varied only with fluid subcooling regardless of jet parameters and that both the pool and impingement nucleate boiling curves at the same subcooling condition were well correlated. The high heat flux portions of the boiling curves with jet exit velocities greater than 10 m/s were corrected for the elevated saturation temperature. A new expression was developed with an interpolation method to construct the partially developed nucleate boiling curve.     

Crystallization fouling of finned tubes during pool boiling: effect of fin density

Bubble characteristics such as density, size, frequency and motion are key factors that contribute to the superiority of nucleate pool boiling over other modes of heat transfer. Nevertheless, if heat transfer occurs in an environment prone to fouling, the very same parameters may lead to accelerated deposit formation due to concentration effects beneath the growing bubbles. This has led to the widely accepted design recommendation to maintain the heat transfer surface temperature below the boiling point if fouling may occur, e.g., in seawater desalination. The present paper aims at investigating the formation of deposits on finned tubes during nucleate pool boiling of CaSO₄ solutions. The test finned tubes are low finned tubes with fin densities of 19 and 26 fins/in. made from Cu–Ni. The fouling experiments were carried out at atmospheric pressure for different heat fluxes ranging from 100 to 300 kW/m² and a CaSO₄ concentration of 1.6 g/L. For the sake of comparison, similar runs were performed with smooth stainless steel tubes. The results show that: (1) the fouling resistance decreases with increasing fin density, (2) fouling on the finned tubes was reduced with increasing nucleate boiling activity and (3) if any fouling layer occurred on the finned tubes it could be removed easily.     

Liquid film circumferential asymmetry prediction in horizontal annular two-phase flow

This study considers the prediction of the degree of asymmetry in the circumferential distribution of the liquid film in the tube cross section of horizontal annular gas–liquid two-phase flow, endemic of the lower region of this flow regime near the stratified-wavy flow transition boundary. Focusing on disturbance waves as the predominant mechanism for transporting the liquid in the annular film from the bottom to the top of the tube to counterbalance the draining effect of gravity, a new prediction method for the degree of asymmetry in the annular liquid film is proposed that outperforms existing correlations. Flow pattern maps for horizontal gas–liquid two-phase flow of frequent use in the design of evaporators and condensers can thus be explicitly updated to account for both symmetric and asymmetric annular flows. The underlying experimental database contains 184 measured liquid film circumferential profiles, corresponding to 1276 local liquid film thickness measurements collected from 15 different literature studies for tube diameters from 8.15 mm to 95.3 mm.