Convective Boiling Heat Transfer Research Articles

Global optimization for the design of shell and tube horizontal thermosyphon reboiler

Shell and tube horizontal thermosyphon reboilers (STHTs) find wide applications in the process industry particularly due to their inherent advantages in handling large duties, high viscosity fluids and low fouled susceptibility. However, current design commercial packages suffer from obvious limitations including extensive user manipulations, trial-and-error necessity, increased human factor influence. To overcome these limitations, we develop in this work a rigorous MINLP model for STHT sizing, by achieving automated selection of detailed exchanger geometries with optimal overall heat transfer coefficient. The corrected minimum cross-flow area in shell-side is incorporated to accommodate full cross-flow patterns in tube bundle, thus handling popular service of X shell without baffle construction. A dependable convective boiling heat transfer model is proposed to obtain the shell-side heat transfer coefficient, validated with Aspen EDR. The critical variable, heat flux, which affects nucleate boiling heat transfer performance, is optimized through a simultaneous equation-solving process using a global solver, reducing excessive unit overdesign. Pressure balance between STHT and distillation column is fully considered by a proposed optimization model incorporating pressure drop and pipe sizing. By employing global optimization techniques and automating the design process, this innovative approach minimizes manual adjustments and provides more accurate and cost-effective designs.

Experimental analysis of convective boiling heat transfer and nanoparticle deposition effect of TiO2-H2O nanofluids in microchannels

The convective boiling heat transfer characteristics of TiO2-H2O nanofluids in microchannels were experimentally investigated in this paper. The effects of the mass flow rate, mass fraction and nanoparticle size on the average flow boiling heat transfer coefficient were examined at heat fluxes of 138.4–220.5 kW/m2. The results have shown that the average flow boiling heat transfer coefficient of TiO2-H2O nanofluids increases with the increase of the mass flow rate and nanoparticle size. When the mass fraction of TiO2-H2O nanofluids ranges from 0.01 % to 0.1 %, the maximum heat transfer efficiency has reached with the mass fraction of 0.025 %. Considering the movement and deposition of the TiO2 nanoparticles in the convective boiling process, the difference of the absorbance value of TiO2-H2O nanofluids and heating surface morphology after boiling were observed. It proved that the distribution density of the nanoparticle deposition gradually increases along the flow direction of the microchannel. Furthermore, the mass fraction of TiO2-H2O has great influence on the nanoparticles deposition morphology and the nucleation condition of the boiling surface.

Effect of contraction on the convective boiling heat transfer of microchannel heat sinks

The present study examines the effect of contraction before straight fin microchannels in the cold plate by using refrigerant R-134a as a working fluid. The experiments compared two test sections with the same geometry and dimension, but one has contraction before the microchannels (CBM), and another one has no contraction before the microchannels (NCBM); the mass flux ranges from 25 to 425 kg/m2⋅s while the heat flux spans from 60 to 300 kW/m2. The inlet flow conditions can be either a fully liquid inlet or with vapor qualities (from 0.1 to 0.45). Improved flow distribution is found with the introduction of the contraction. Based on the mass flux and irrespective of the inlet conditions, the test results indicated that the performance of the CBM shows a superior heat transfer coefficient (HTC) by about 3 ∼ 30 % than that of NCBM. Nonetheless, the pressure drop of CBM is lower than NCBM when the inlet is fully liquid. Conversely, the trend of pressure drop is reversed with a vapor-quality inlet. For the fully liquid inlet, the CBM contains a much lower pressure drop by 22 % compared to NCBM at a low mass flux of 36.2 kg/m2⋅s. However, the reduced pressure drop has narrowed to 3 % at a mass flux of 402.8 kg/m2⋅s. Flow visualizations were performed, and it was found that the CBM is free from flow reversal, and the fluctuations in temperature or pressure drop are much less pronounced. By contrast, the NCBM design shows flow reversal and sluggish bubble phenomena.

The characteristics of forced convective boiling heat transfer of ammonia in microchannel heat exchanger

The characteristics of forced convective boiling heat transfer of ammonia in microchannel heat exchanger

A Simple Correlation for Horizontal Microfin Tube Convective Boiling

Abstract This paper presents a simple correlation containing nine dimensionless parameters to predict the local convective boiling heat transfer within a microfin tube that was developed from a dataset that included a wide range of refrigerants, flow conditions, and hydraulic diameters. Flow boiling measurements from 36 studies were gathered to produce a dataset containing 29 refrigerants and 15 different hydraulic diameters ranging from 1.8 mm to 6.7 mm. The database was adjusted so that the definition of the Nusselt number, the Reynolds number, the heat flux, and heat transfer coefficient were all defined in the same way for each study. In addition, values for the most recent fluid properties were used for the calculation of dimensionless parameters that were used in the new correlation. The new correlation for microfin convective boiling predicted the measurements from the database to within ±10.3% for pure-refrigerants and azeotropic mixtures and ±12.3% for zeotropic mixtures.

An experimental investigation on convective boiling heat transfer of R-454B with lubricant oil of POE-32 or POE-68 mixture in a horizontal smooth tube

The effect of lubricant (POE-32 and POE-68) on convective boiling heat transfer is experimentally investigated inside a smooth horizontal copper tube having an outer diameter of 9.52 mm. R-32, R-454B, and R-410A are used as working fluids. The oil mass concentration ranges from 0% to 6%. For pure refrigerant, the heat transfer coefficient (HTC) of R-454B is about 4–5% higher than those of R-32 and R-410A. The presence of lubricant oil deteriorates the HTC and increases the pressure drop appreciably, and it becomes more pronounced with oil concentrations. The HTC increases with the increase in the heat flux, but there is no definite tendency regarding the effect of lubricants on HTC subject to heat flux. The deteriorations of HTC become more severe at high vapor quality with impairment of more than 40%. With a 3% oil concentration, the level-off for the HTC of the R-32/POE-32 mixture occurs at a vapor quality of 0.7 and is 0.8 for R-410A/POE-32, and R-454B/POE-32, respectively. For vapor quality ranges from 0.1 to 0.6 at the 3% oil concentration, the effect of the POE-68 oil on HTC is more pronounced than that POE-32 in the R-32/oil mixture. For the R-454B/oil mixture, the effect of lubricant on HTC is lessened.

Experimental investigation of the annular flow caused by convective boiling in a heated annular channel

An experimental investigation was performed on the convective boiling heat transfer of the annular flow in concentric and eccentric annuli with a central heating rod in an unheated tube. Advanced measurement techniques including laser-induced fluorescence (LIF) and confocal chromatic sensor (CCS) were applied to acquire the dynamics and instabilities of the liquid thin film on the tube. The boundary conditions were as follows: heating rod heat flux from 167 kW/m2 to 201 kW/m2 and mass flow rate from 58 g/s to 155 g/s. The annuli which the flow occurred has a hydraulic diameter of 15.5 mm. As indicated previously by the research on isothermal annular flow in bare tubes, the vapor superficial velocity is the primary factor that influences the liquid film dynamics including the base film thickness and wave amplitude. The liquid film thickness in the eccentric geometry was observed to be constant for liquid superficial velocities from 0.15 m/s to 0.34 m/s and vapor superficial velocities from 6.5 m/s to 13.2 m/s. The heat transfer coefficient ranged from 2.734 kW/m2∙K to 4.279 kW/m2∙K for the concentric geometry and 2.063 kW/m2∙K to 3.096 kW/m2∙K for the eccentric geometry. An increasing trend of the heat transfer coefficient was observed as the liquid superficial velocity increased, whereas the reverse trend was observed for the vapor superficial velocity. When the measured heat transfer coefficient was exceptionally low (< ~2.0 kW/m2∙K), the boiling condition was assumed to be the dry out. An extended range of boundary conditions reported in this study (from the annular flow to the dry out regime) can provide useful data sets for validating computer codes. In particular, the detailed information on liquid film thickness, wave characteristics, and heat transfer coefficient provided in the present paper can help develop annular flow boiling models in computational fluid dynamics.

On the scaling of convective boiling heat transfer coefficient

Flow boiling has been widely used during the last century, but the key mechanisms controlling the heat transfer process still remain elusive. In the particular case of convective flow boiling inside a heated pipe, most of the existing correlations have been proposed assuming an enhancement factor to the liquid Reynolds number. In this work, we show that during convective boiling heat transfer, the vapor Reynolds number plays a dominant role which has been overlooked. By comparing experimental data in this work and from the literature, we show that the enhancement factor to the liquid Reynolds number does not appropriately correlate the heat transfer coefficient. Further, it can be shown that most of the proposed enhancement factors can be re-written in terms of the explicit contribution of the ReV. In particular, at high qualities, the influence of ReL becomes negligible.

Convective boiling heat transfer under microgravity and hypergravity conditions

The present paper concerns an experimental investigation of the gravitational effects on subcooled and saturated flow boiling heat transfer in a vertical 3.75 mm diameter aluminum tube, using perfluorohexane as working fluid. Experimental results obtained during parabolic flights are compared with data gathered in the same test facility for normal gravity. Results were obtained for mass velocities of 133, 252, 377 and 428 kg/m2s, heat fluxes up to 106 kW/m2 and vapor qualities up to 0.28. The assessments of prediction methods from literature for flow boiling heat transfer proposed for normal gravitational conditions were evaluated through comparisons of calculated and experimental results. In general, for either a mass velocity of 428 kg/m2s or a vapor quality lower than -0.2, the gravitational effects on the heat transfer coefficient were found as negligible. For lower mass velocities and saturated flow boiling, higher heat transfer coefficients were observed under hypergravity conditions. This behavior was associated to the fact that higher gravitational forces favor bubbles detachment. The method of Kanizawa et al. [1] provided reasonable predictions of the saturated flow boiling results under microgravity and hyper gravity conditions, predicting 100% of the data within an error band of ±30%.

Experimental Evaluation of Critical Heat Flux in Downward-Facing Boiling on SS304 L Flat Plate Relevant to In-Calandria Retention in PHWRs

Abstract Retention of corium inside the calandria vessel (CV) by externally cooling it by calandria vault water is essential to mitigate severe accidents in pressurized heavy water reactor (PHWR). The thermal failure of CV can be prevented by effective decay heat removal on the outer surface of CV using vault water, which depends on the heat transfer behavior from the outer surface of CV to the vault water. Determination of limiting heat removal capability of vault subcooled water through outer surface of CV is very important. Since the CV has a very large diameter and length, the bottom most part of the CV almost behaves as a flat plate with downward natural convection boiling heat transfer. The natural convection heat transfer is lesser on the flat surface as compared to the curved surface of the CV. Thus, the critical heat flux (CHF) on the flat surface under downward boiling condition is the limiting CHF of the CV under external surface boiling scenario. In order to estimate CHF in this configuration with local boiling, experiments were carried out on a downward facing SS304 L flat plate simulating the conditions of CV of 700 MWel Indian PHWR. The pool boiling CHF obtained in this study is also compared with other earlier works.

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.

Evaluation of forced convective boiling heat transfer with layered parallel microchannels

Evaluation of forced convective boiling heat transfer with layered parallel microchannels

Flow-pattern-based experimental analysis of convective boiling heat transfer in a rectangular channel filled with open-cell metallic random porous media

This paper reports a visualization study on the flow boiling of R245fa in a rectangular channel filled with an open-cell random porous structure made of copper. The boiling heat transfer of the two-phase vertical upward flow inside the channel was measured while the refrigerant was asymmetrically heated by a hot-water-source channel. An experiment was conducted with a constant saturation pressure of 5.9 bar, inlet vapor quality of 0.05–0.99, channel heights of 3 and 5 mm, and mass flux of 133–300 kg/m2 s. The results are compared with those of a plain channel in the same operating conditions. The two-phase flow characteristics are discussed along with the trend of the boiling heat transfer coefficient and mean vapor quality according to the flow patterns obtained with and without the metallic porous media. A non-dimensional analysis was also conducted for comparison.

Experimental investigation of flow boiling heat transfer performance in zigzag microchannel heat sink for electronic cooling devices

This study proposes an improved microchannel heat sink with offset zigzag cavities to enhance boiling performance. Flow and heat transfer characteristics are experimentally investigated via a microscale boiling heat transfer visualization system using acetone as the working fluid. At mass fluxes of 285, 402, and 684 kg/(m2·s) and heat fluxes of 1.31–80.26 W/cm2, the effects of heat and mass fluxes on flow pattern, heat transfer, wall temperature, and pressure drop of zigzag microchannel heat sink are analyzed and compared with conventional rectangle microchannel heat sink. Results show that the fluctuating amplitude first increases and then decreases with the increment of heat fluxes, thereby causing a quasi-steady and unstable boiling with small fluctuations. Instability becomes increasingly severe for the nucleate boiling area moves toward the inlet and flow reversal intensity increases with heat fluxes under low heat flux. With heat flux increased furtherly. The proportion of steam in the channel increases to reduce fluctuations. Compared with the rectangular microchannel, the zigzag microchannel evidently enhances heat transfer characteristic with lower wall temperature at the onset of nucleate boiling (ONB), higher heat transfer coefficient (HTC) and critical heat flux (CHF) and improves boiling stability by suppressing flow reversal and reducing two-phase pressure drop. The mechanism of zigzag microchannel enhancement boiling performance can be attributed to (1) the improved disturbance of the working fluid, which enhances forced convective boiling heat transfer and mitigates bubble coalescence and flow reversal strength; (2) the continuously developing liquid film and improved wettability maintain a stable liquid film evaporation and delay partial dry out. Increase in mass flux can improve CHF and reduce pressure drop albeit with high wall temperature of ONB. The local HTC is enhanced, except for the low heat fluxes, because the bubbles are condensed by liquid and cannot grow to absorb heat at low heat fluxes.

Convective boiling heat transfer of methanol – Hydrogen peroxide solutions in a microchannel evaporator

To provide uniform mixture of methanol vapor, steam and oxygen in a methanol reformer, the present study investigates the boiling heat transfer performance and two-phase flow of methanol and 50 wt% hydrogen peroxide mixture in an microchannel evaporator with V-shaped micro diffusers for the entrance region and 52 parallel diverging channels. The decomposition of hydrogen peroxide may produce oxygen and significant amount of heat making the convective boiling of the mixture extraordinary. Experiments of three volume ratios (C) of hydrogen peroxide to methanol of 1/2, 1/3, 1/4 and three flow rates, 0.18, 0.36, and 0.54 ml/min are conducted and analyzed. The experimental results reveal that the addition of hydrogen peroxide can significantly enhance boiling heat transfer as well as the critical heat flux possibly due to the Marangoni convection caused by concentration gradient, but the enhancement is limited by the introduction of decomposition heat internally in the working fluid. The oxygen and enhanced evaporation due to decomposition may make the boiling two-phase flow unstable and deteriorate the heat transfer. Consequently, the mixture with C = 1/3 showing the best heat transfer coefficient may be the optimized working fluid for a microchannel evaporator to serve the purpose of this study.

Simulation of natural convection boiling heat transfer for refrigerant R-134a flow in a metal foam filled vertical tube

Natural convection boiling heat transfer for Refrigerant R-134a flow in a metal foam filled vertical tube is numerically investigated in the present work by using the Multiphase Mixture Model MMM at constant heat flux for both transient and steady state behavior under different parameters. The effect of changing imposed heat flux, porosity, PPI (pore per inch), and tube diameter was analyzed. It is found that liquid saturation will reach its minimum value at the exit, and the dry out point will be seen always at the 2nd half of the tube length. Results also show that smaller tube diameter and higher porosity for the same PPI caused a higher temperature variation over the entire metal foam tube. Comparisons with previous works show good agreement.

Phenomenological correlations for two phase heat transfer and pressure drop performance of electrohydrodynamic horizontal convective boiling of dielectric fluids

EHD is an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids whose performance varies widely in the literature for even the same geometry and fluids. As such, the performance of EHD convective boiling devices remains largely unpredictable. Two empirical EHD convective boiling heat transfer coefficient correlations exist in the literature with good performance when compared to their exact test dataset and geometry, however they have been shown to have poor correlations in predicting the performance for external test datasets. In this paper, a phenomenological approach is taken in the development of a new EHD convective boiling heat transfer coefficient correlation and a novel EHD convective boiling pressure drop correlation. The performance correlations are based on widely-used free-field convective boiling correlations for heat transfer coefficient and pressure drop with phenomenological enhancement factors based on the two phase flow pattern as predicted using the EHD two phase flow pattern map which accounts for the effect of electric field strength on flow pattern redistribution, as described in a previously published paper (Nangle-Smith and Cotton, 2018). A review of the EHD convective boiling experimental literature was conducted to determine confounding factors that can impact the large variance in the reported performance. Flow pattern and applied heat flux were identified as common parameters not maintained constant in the experimental data. Two datasets, including data from the present study, in which flow pattern and applied heat flux are maintained constant were used in the development of the performance correlations. Considerable improvement over previous EHD convective boiling performance correlations was found both in error and the physical significance of coefficients. The correlations were developed for thermodynamic qualities in the range of 20–60% below the onset of dryout or mist flow patterns and for applied heat fluxes < 30 kW/m2. Furthermore, test conditions in this study were chosen to focus on the dielectrophoretic effect and therefore studies with significant electrophoresis were not included. Thus, the correlation is limited to positive applied voltages, saturated flow boiling conditions, and field strengths below the onset of charge injection. The authors recommend this approach for EHD two phase performance correlation development as it is more mechanistic, is analogous to the state-of-the-art approaches for free-field two phase performance, and is likely to yield more accurate models as more experimental data becomes available.

Low mass flux upward vertical forced flow boiling of HFE7000

Forced convective boiling experiments with HFE7000 as the working fluid were conducted for upward flow in a vertical tube at 122 kPa (1.2 bar), inlet subcooling of 2 K and mass flux ranging between 50 and 150 kg/m2 s. The experimental facility consisted of a smooth 8 mm inside diameter optically transparent sapphire tube with overall length of 120 mm. A central 80 mm long section was coated with optically transparent and electrically conductive Indium Tin Oxide (ITO) functioning as a heater to facilitate boiling whilst allowing for visual observation of the two-phase flow. Tests were performed by fixing the flow rate and progressively increasing the heat flux. The results show that after the onset of nucleate boiling, the flow regime is bubbly and increasing the heat flux acts to increase the boiling heat transfer coefficient due to increased nucleation site density, bubble frequency and size. This results in a linear increase in the boiling curve. However, once slug flow is established, the slope of the boiling curve increases indicating that this flow regime augments the heat transfer positively. Overall, the boiling curves were insensitive to mass flux for the range tested. However, the inflection on the boiling curve, associated with flow regime transition from bubbly to slug, was observed to be sensitive to mass flux. The Chen convective boiling heat transfer correlation was compared with the measured data showing reasonable agreement. A modified Chen correlation has been proposed for low mass flux convective boiling which predicts the measured data more accurately.

Horizontal convective boiling of R1234yf, R134a, and R450A within a micro-fin tube

This paper presents local convective boiling heat transfer and Fanning friction factor measurements in a micro-fin tube for R134a and two possible low global warming potential (GWP) refrigerant replacements for R134a, namely R1234yf and R450A. Test section heating was achieved with water in either counterflow or in parallel flow with the test refrigerant to provide for a range of heat fluxes for each thermodynamic quality. An existing correlation from the literature for single and multi-component mixtures was shown to not satisfactorily predict the convective boiling measurements for flow qualities greater than 40%. Accordingly, a new correlation was developed specifically for the test fluids of this study so that a fair comparison of the heat transfer performance of the low GWP refrigerants to that of R134a could be made. The new correlation was used to compare the heat transfer coefficient of the three test fluids at the same heat flux, saturated refrigerant temperature, and refrigerant mass flux. The resulting example comparison, for the same operating conditions, showed that the heat transfer coefficient of the multi-component R450A and the single-component R1234yf were, on average, 15% less and 5% less, respectively, than that of the single-component R134a. Friction factor measurements were also compared to predictions from an existing correlation. A new correlation for the friction factor was developed to provide a more accurate prediction. The measurements and the new models are important for the evaluation of potential low-GWP refrigerants replacements for R134a.

Experimental investigation of boiling heat transfer in helically coiled tubes at high pressure

Helically coiled once-through steam generators have been used widely during the past several decades for small nuclear power reactors. The boiling heat transfer characteristics of helically coiled tubes are important to optimal design of helically coiled steam generators. In the present work, various experiments with helically coiled tubes are performed to investigate the boiling heat transfer characteristics at high pressure with water as the working medium. Six test sections are made of thin walled stainless steel tubes having inner diameters of 12.5 mm and 14.5 mm. The system pressure is varying from 2 to 8 MP. The effects of heat flux, mass flux and system pressure on boiling heat transfer behavior are discussed in detail. Increase in heat flux increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient. However, heat flux has no influence on saturated convective boiling heat transfer coefficient. Mass flux increases the saturated convective boiling heat transfer coefficient. Increase in system pressure increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient but decreases the saturated convective boiling heat transfer coefficient. For the investigated experimental conditions, the results show that the Baburajan's correlation gives the best prediction in subcooled boiling region and the Gungor and Winterton's correlation predicts well with the experimental data in saturated boiling region. For the investigated experimental conditions, the results also indicate that the helically coiled tubes have similar performance of heat transfer in boiling regions compared with straight tubes.