Two-phase Convective Heat Transfer Research Articles

Experimental and analytical study of the hydrodynamic and single and two-phase convective heat transfer performance of flexible PDMS microchannels with micropillar arrays

Various copper and silicon based thermal management systems are used in the cooling of electronics. However, the rigid nature of these materials along with their high thermal and electrical conductivity pose a difficulty in developing direct contact embedded flexible cooling systems that can offer robust cooling performance. The low density, thermal stability, chemical inertness, and electrical insulation of Polydimethylsiloxane (PDMS) make it an ideal material to develop lightweight direct contact thermal management systems for electronics. Its ease of fabrication with tunable flexibility provides the opportunity to go beyond traditional electronics and develop advanced active and passive thermal management systems for a wide–range of applications in foldable and wearable electronics, liquid cooling garments, microgravity, and electric motors. In this study, a flexible PDMS based microchannel with micropillar arrays, which enhance the thermal performance of the device through capillary-assisted flow, has been developed. The hydrodynamic and convection heat transfer performance of three PDMS wick pillar geometries, ranging from a porosity of 0.8–0.91, are investigated and compared under single-phase and two-phase conditions. Dielectric coolant FC-3283 is employed and permeability measurements are made for mass fluxes ranging from 53 kg/m2 s to 369 kg/m2 s. Given its conformability, the device demonstrates a deviation from Darcy’s Law, within the laminar regime, with an increasing permeability with mass flux at the rate of ∼0.5–0.8 Darcy/(kg/m2 s). A semi-analytical model has been developed and reported to quantify the conformability of the device. The heat transfer performance is experimentally evaluated using the same dielectric fluid for mass flux ranging from 105 kg/m2s to 420 kg/m2s with heat fluxes ranging from 1.5 W/cm2 to 16 W/cm2. Heat transfer coefficients of up to 7000 W/m2 K are observed, which are comparable to copper and silicon microchannels. The effect of porosity on the single phase thermal performance has been evaluated against the pumping power to provide a basis for thermal management system design. High-speed imaging is performed to study the two-phase flow characteristics to provide insight into the vapor formation and removal.

Hybrid or mono nanofluids for convective heat transfer applications. A critical review of experimental research

Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for two-phase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.

Experimental investigation of single- and two-phase convective heat transfer in slender rectangular 180° return bends

The convective heat transfer characteristics of single-phase water flows and two-phase air-water flows through a slender 180° return bend with a rectangular cross-section are investigated. A moderately curved section is considered with a curvature ratio around 58, a hydraulic diameter of 3.9 mm and an aspect ratio of 1.067. The focus of this work is to investigate if there is a recognizable difference in heat transfer, respectively Nusselt numbers, between the outer (convex) and inner (concave) curved surface of the duct. An approach that considers the conjugated heat transfer is applied to adequately investigate this difference. Additionally the impact of the secondary gas-phase on the heat transfer characteristics is discussed.

Two-phase convection heat transfer correlations for liquid hydrogen pipe chilldown

Recently, heat transfer correlations based on liquid nitrogen (LN2) and liquid hydrogen (LH2) pipe quenching data were developed to improve the predictive accuracy of lumped node codes like SINDA/FLUINT and the Generalized Fluid System Simulation Program (GFSSP). After implementing these correlations into both programs, updated model runs showed strong improvement in LN2 pipe chilldown modeling but only modest improvement in LH2 chilldown modeling. Due to large differences in thermal and fluid properties between the two fluids, results indicated a need to develop a separate set of LH2-only correlations to improve the accuracy of the simulations. This paper presents a new set of two-phase convection heat transfer correlations based on LH2 pipe quenching data. A correlation to predict the bulk vapor temperature was developed after analysis showed that large departures from thermal equilibrium between the liquid and vapor phases occurred during film boiling of LH2. Implemented in a numerical model, the new correlations achieve a mean absolute error of 19.5 K in the predicted wall temperature when compared to recent LH2 pipe chilldown data—an improvement of 40% over recent GFSSP predictions. This set of correlations can be implemented in simulations of the transient LH2 chilldown process. Such simulations are useful for predicting the chilldown time and boil-off mass of LH2 for applications such as the transfer of LH2 from a ground storage tank to the rocket vehicle propellant tank, or through a rocket engine feedline during engine startup.

Numerical Investigation into the Two-Phase Convective Heat Transfer within the Hold of an Oil Tanker Subjected to a Rolling Motion

A crude oil tanker usually encounters a rolling motion during sea transportation, which leads to rotational movement and sometimes a sloshing of the liquid hold. This rolling-induced body motion seriously affects the thermal and hydraulic behavior of the liquid hold, which then affects the heating process and heat preservation of the tanker. Clarification of the involved thermal and hydraulic characteristics is the basic requirement for establishment of a scientific heating scheme and heat preservation method. A two-phase 3D model considering the free liquid surface and non-Newtonian behavior of the fluid was established for the thermal calculation of the liquid holds in oil tankers. The thermal and hydraulic characteristics of the liquid hold were investigated under different combinations of dimensionless parameters, and the combined effect of rolling and fluid non-Newtonian behavior was investigated. It was found that rolling intensifies the heat transfer based on the combination of the Richardson number (Ri) and the rotation-strength number (ω*), and non-Newtonian behavior of the fluid effectively affects the heat transfer in a rolling motion. This research is expected to provide a reference for design and optimization of the heating and heat preservation method for oil tanker operation.

Flow regimes and convective heat transfer of refrigerant flow boiling in ultra-small clearance microgaps

Understanding two-phase convective heat transfer under extreme conditions of high heat and mass fluxes and confined geometry is of fundamental interest and practical significance. In particular, next generation electronics are becoming thermally limited in performance, as integration levels increase due to the emergence of ‘hotspots’ featuring up to ten-fold increase in local heat fluxes, resulting from non-uniform power distribution. An ultra-small clearance, 10μm microgap, was investigated to gain insight into physics of high mass flux refrigerant R134a flow boiling, and to assess its utility as a practical solution for hotspot thermal management. Two configurations – a bare microgap, and inline micro-pin fin populated microgap – were tested in terms of their ability to dissipate heat fluxes approaching 1.5kW/cm2. Extreme flow conditions were investigated, including mass fluxes up to 3000kg/m2s at inlet pressures up to 1.5MPa and exit vapor qualities approaching unity. Dominant flow regimes were identified and correlated to two phase heat transfer coefficients which were obtained using model-based data reduction for both device configurations. The results obtained were compared to predictions using correlations from literature, with the maximum heat transfer coefficient reaching 1.5MW/m2K in the vapor plume regime in the case of the finned microgap.

A comprehensive, mechanistic heat transfer modeling package for dispersed flow film boiling – Part 1 – Development

Accurate predictions of Dispersed Flow Film Boiling (DFFB) heat transfer are necessary during both the blowdown and reflood portions of a Loss-of-Coolant-Accident to ensure the correct initial fuel rod temperature distribution for the beginning of the reflood phase and ultimately, determining the peak cladding temperature. Numerous correlative, phenomenological, and mechanistic DFFB heat transfer models have been published; however, most of these models make simplifying assumptions that adversely impact their accuracy or are too computationally intensive to implement into current reactor safety codes. A comprehensive, mechanistic heat transfer modeling package has been developed to account for the six interrelated heat transfer paths in DFFB. Highlights of the model include a Lagrangian subscale trajectory based dry contact heat transfer model and a novel method of determining the two-phase convective heat transfer enhancement due to dispersed droplets intermittently altering the local vapor temperature distribution.

Two-phase Convective Flow in Microchannel with Nanoporous Coating

Effect of nanoporous layer on convective heat transfer performance of microchannel has been investigated experimentally. Experimental studies were carried out in a microchannel of 672μm hydraulic diameter using de-ionized (DI) water as the coolant. Nanoporous layer was developed on the heat transfer surface of the microchannel using electrophoretic deposition of Al2O3 nanoparticles. Both single-phase and two-phase convective heat transfer experiments were performed at different mass flux. Results from the microchannel with bare surface are used as the base line data. Critical heat flux (CHF) increases for the coated surface; up to 45% enhancement in CHF was observed, whereas heat transfer coefficient (HTC) was observed to reduce for the coated surface compared to the bare surface. This observation is in contradiction with several previous reports on pool boiling where significant enhancement in HTC is reported for the nanoporous coating. Possible reason of this contradiction is also explained in this report by performing molecular dynamics simulation.

Calculation of Two-Phase Convective Heat Transfer Coefficients of Cryogenic Nitrogen Flow in Plate-Fin Type Heat Exchangers

The two-phase convective heat transfer coefficients for nitrogen inside the flow path of plate-fin type heat exchangers operating at cryogenic temperatures are calculated using CFX Release 13.0. Using a homogeneous two-phase model, the governing equations are solved to find pressure, velocity, and enthalpy distributions for three types of fin geometries: plain, wavy, and serrated. The results are further processed to evaluate the wall shear stress and heat flux, which in turn yield the friction coefficients and convective heat transfer coefficients. The coefficients are presented as functions of system pressure, flow rate, and local quality. The results can be used for the design of plate-fin type exchangers with the same fin configurations and operating conditions as the calculation.

Investigation of grid-enhanced two-phase convective heat transfer in the dispersed flow film boiling regime

A two-phase dispersed droplet flow investigation of the grid-enhanced heat transfer augmentation has been done using steam cooling with droplet injection experimental data obtained from the Penn State/NRC Rod Bundle Heat Transfer (RBHT) facility. The RBHT facility is a vertical, full length, 7×7-rod bundle heat transfer facility having 45 electrically heated fuel rod simulators of 9.5mm (0.374-in.) diameter on a 12.6mm (0.496-in.) pitch which simulates a portion of a PWR fuel assembly. The facility operates at low pressure, up to 4 bars (60psia) and has over 500 channels of instrumentation including heater rod thermocouples, spacer grid thermocouples, closely-spaced differential pressure cells along the test section, several fluid temperature measurements within the rod bundle flow area, inlet and exit flows, absolute pressure, and the bundle power. A series of carefully controlled and well instrumented steam cooling with droplet injection experiments were performed over a range of Reynolds numbers and droplet injection flow rates. The experimental results were analyzed to obtain the axial variation of the local heat transfer coefficients along the rod bundle. At the spacer grid location, the flow was found to be substantially disrupted, with the hydrodynamic and thermal boundary layers undergoing redevelopment. Owing to this flow restructuring, the heat transfer downstream of a grid spacer was found to be augmented above the fully developed flow heat transfer as a result of flow disruption induced by the grid. Furthermore, the presence of a droplet field further enhanced the heat transfer as compared to single phase conditions. From the RBHT steam cooling with droplet injection data, it was found that a second-stage augmentation occurs under wet grid conditions at a distance of approximately 10 diameters downstream of the grid. This second-stage augmentation, which is a direct consequence of a sharp increase of the droplet interfacial area due to the breakup of liquid ligaments downstream of the grid, was not observed under dry-grid conditions nor was it observed in single-phase steam cooling tests. It was also found that the general practice of classifying a grid as wet or dry based solely on the thermocouple temperature is insufficient, as a large scatter was observed in the data.

Effect of Cu–Al2O3 nanocomposite coating on flow boiling performance of a microchannel

Convective heat transfer performance of Cu–Al2O3 nanocomposite coated surface has been investigated experimentally for its potential use in heat transfer equipment operating in corrosive environment. Experimental studies were carried out in a bottom surface heated single microchannel of 672 μm hydraulic diameter using de-ionized (DI) water as the coolant. Thin nanocomposite coating was developed on the bottom surface of the microchannel using electrocodeposition technique. Both single-phase and two-phase convective heat transfer experiments were performed at different mass flux. The results from microchannel with bare Cu surface are used as the baseline data. Cu–Al2O3 nanocomposite coating has been found to enhance single-phase heat transfer rate marginally, whereas in the two-phase regime, the enhancement was ∼30% to ∼120% depending on the flow rate and surface temperature with an additional pressure drop penalty of less than ∼15%. CHF also improves for the coated surface by ∼35% to ∼55%.

Enhanced flow boiling in a microchannel with integration of nanowires

Convective heat transfer performance of a micro-channel with copper nanowires (CuNWs) coatings has been investigated experimentally. Experimental studies were carried out on a bottom surface heated single micro-channel of 672 μm hydraulic diameter using de-ionized (DI) water as coolant. Nanowires were directly grown on the bottom surface of the micro-channel using electrochemical deposition technique. Both single-phase and two-phase convective heat transfer experiments were performed at different mass flux and different degree of sub-cooling. The results from microchannel with bare surface are used as the baseline data. CuNWs coatings have been found to enhance single-phase heat transfer rate by up to ∼25%, whereas in the flow boiling regime, the enhancement was up to ∼56% with a pressure drop increase by ∼20% in the single-phase regime. The obvious change of pressure drop in the fully developed boiling regime was not observed.

Effect of a Hydrophobic Coating on the Local Heat Transfer Coefficient in Forced Convection under Wet Conditions

An estimation technique of the local heat transfer coefficient, based on the solution of the 2-D inverse heat conduction problem, has been adopted in order to investigate the effect of the surface wettability on the two-phase convective heat transfer in a dehumidifying process. The convective heat transfer coefficient distribution has been restored on aluminum plates coated with a hydrophobic oleic film on which the dropwise condensation of the water vapor carried by a humid air turbulent stream occurs. The refinement of the technique in relation to its ability of capturing the heat transfer local capability of the surface enables the appraisal of the heat transfer augmentation due to the hydrophobic surface coating.

Two-Phase Convective Heat Transfer in Miniature Pipes Under Normal and Microgravity Conditions

Detailed numerical simulations have been performed to study the effect of flow orientation with respect to gravity on two-phase flow heat transfer (without phase change) in small diameter pipes. The Nusselt number distribution shows that the bubbly, slug, and slug-train regimes transport as much as three to four times more heat from the tube wall to the bulk flow than pure water flow. The flow blockage effect of the inclusions results in a circulating liquid flow superimposed on the mean flow. For upflow, the breakup into bubbles/slugs occurs earlier and at a higher frequency. The average Nusselt numbers are not significantly affected by the flow orientation with respect to gravity. A mechanistic heat transfer model based on frequency and length scale of inclusions is also presented.

A Finite Element Computational Method for Gas Hydrate. Part II: Application

A finite element model has been applied to the dissociation of gas hydrates in porous media by hot water injection. In the first step, it was applied to calculate the temperature and saturation profiles for a two-phase convective heat transfer case. The second step involved hydrate thawing calculations with release of water and gas. Realistic results were obtained in predicting the temperature, pressure, and water saturation profiles in the hydrate medium with respect to time, when subjected to hot water injection at high pressure in a wellbore. Furthermore, the thaw front progression calculated by this scheme exhibits stable and satisfactory behavior with respect to time.

A Computational Scheme for Fluid Flow and Heat Transfer Analysis in Porous Media for Recovery of Oil and Gas

A finite-element scheme has been formulated, which is capable of solving transient analysis of both conductive and convective heat transfers due to fluid flow in porous media. The model also includes the latent heat effect to consider the phase change aspect of a frozen medium. To test the validity of the model, it was applied to six cases for which analytical solutions are available. The test cases cover (i) single-phase fluid flow through porous media, (ii) radial conduction with and without phase change, (iii) conductive and convective heat transfer in an aquifer, and (iv) two-phase immiscible flow in porous media. In all these cases, good agreement with analytical solutions are observed validating the computational scheme. This computational scheme should be useful in solving frozen ground problems, thermal stimulation technique for natural gas recovery from hydrates, and single-phase and two-phase convective heat transfer problems in enhanced oil recovery scheme in petroleum engineering.

Heat transfer characteristics and mechanisms along entire boiling curves under steady-state and transient conditions

The paper presents a survey of results found by the authors and their groups during recent years. An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wetting fluids and fluids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean. However, even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run. The situation is different under transient conditions: heating and cooling transients yield different curves even on clean surfaces. Measurements with micro-optical probes give an insight in the two-phase dynamics above the heating surface in the different boiling regimes. Temperature signals from microthermocouples at the surface and the optical probe data lead to some conclusions on the physics of boiling in different regimes such as the macrolayer configuration, the dry-spot size and dynamics, the nucleation site density etc. The high density of active nucleation sites in the region between fully developed nucleate boiling and critical heat flux (CHF) and the resulting highly turbulent two-phase boundary layer let us conclude that the strong increase of heat flux in heating transients and vice versa in a cooling process is mainly due to the intensive two-phase convection heat transfer from the wall to the bulk and not due to inertia effects in the two-phase structure dynamics. The latter is not significantly affected by temperature transients. An interfacial-area-density model is proposed. It needs data from the optical probes and enables the prediction of entire boiling curves by employing only one fitting parameter. Furthermore the concept of a reaction–diffusion model is presented to predict CHF. Here the triggering of CHF is due to an instability of dry spots on the heating surface. Data from microthermocouples and optical probes are needed to evaluate this model. Many aspects of the extremely complex mechanisms of boiling are still not sufficiently understood. The problems should be tackled from both the experimental and the theoretical end and both approaches should be closely linked.

Immiscible two-component two-phase gas?liquid heat transfer on shell side of a TEMA-F heat exchanger

Systematic studies of heat transfer on shell side of a horizontal TEMA-F heat exchanger have been conducted using glycerine-water solution and air mixture. The effects of gas flux, liquid flux as well as flow pattern on heat transfer were studies in detail. The experimental data of two-phase convection heat transfer coefficient on stratified flow, intermittent flow and annular flow were correlated using an enhancement model. The correlation can be recommended for shell-and-tube heat exchanger design.

An experimental study of gas–liquid flow in a narrow conduit

This paper reports an experimental study of non-boiling air–water flows in a narrow conduit (diameter 1.95 mm). Results are presented for pressure drop characteristics and for local heat transfer coefficients over a wide range of gas superficial velocity (0.1–50 m s−1), liquid superficial velocity (0.08–0.5 m s−1) and wall heat flux (3–58 kW m−2). For a given liquid flow rate, the data exhibit sudden changes in pressure drop and, to a lesser extent, in heat transfer characteristics as the gas flow is increased. These events are believed to correspond to flow transitions, from bubbly, to intermittent slug, to annular flow. Overall, the pressure drops in these diabatic non-boiling two-phase flows can be estimated with good accuracy using correlations developed for adiabatic conditions. The heat transfer results are, on average, reasonably well described by the two-phase convective heat transfer components of flow-boiling correlations but there is considerable scatter in some cases.

A method of solving the moving boundary heat transfer problem in plasma sprayed particles

A fast and novel method for calculating the transient two-phase convection and conduction heat transfer of particles in a high-temperature flame or plasma stream is proposed. The computational requirement of this method is small in comparison to other finite-difference numerical methods. Estimates of the required melting times of particles of different sizes can be readily obtained as functions of external flame or plasma temperature and flow velocities. With this method, the non-uniform radial temperature distributions of the inner solid core and the outer liquid annulus of the particle and the solid-liquid boundary are calculated using iterated time-dependent trial functions. Thermal and viscous properties of the particle-plasma flow interaction are taken into account in the calculations.