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This paper reviews recent advances in pool and flow boiling of dilute emulsions, including initial theories on the boiling mechanisms. The current consensus is that pool boiling is initiated on nucleation sites on the surface and that increased heat transfer coefficients are a result of mixing produced by vapor bubbles in the thermal boundary layer. Visualization of the boiling process substantiates this conclusion. Current understanding of the influence of various quantities on the convective heat transfer coefficient is enumerated with respect to the underlying thermodynamics and energy transport. Prior research on flow boiling is limited in comparison, and much more needs to be done with respect to the boiling and enhancement mechanism. Various models for boiling of emulsions are discussed, including a RANS model that incorporates droplet contact with the heated surface, collisions between droplets and bubbles, and chain boiling due to collision during the boiling process. Key modeling elements are how interphase transport and droplet interactions affect nucleation rates and droplet behavior within the viscous and thermal boundary layers at and near a heated surface. It is clear now that boiling of a suspended phase with a lower saturation temperature than the continuous phase can produce marked increases in overall heat transfer coefficients. A re-conceptualization of two-phase heat exchange equipment is a future possibility, and various applications are identified. Boiling of dilute emulsions may be an attractive cooling strategy when restrictions on size, weight and pumping power must be addressed, such as in thermal management of avionics systems.

Granular filters are a promising technology in hot gas clean-up for coal-based power generation systems. Numerical simulations are presented for the filtration characteristics of randomly packed granular filters. The effect of filter height is first investigated to obtain a unit height which can predict filtration characteristics of the entire filter, and it is found that the unit height should be at least 10 times the granule diameter. The filtration performance, including filtration efficiency, pressure drop and deposition uniformity are analyzed, and the effect of gas velocity, particle diameter and granule diameter are then investigated. The results show that the pressure drop increases rapidly with the increase of gas velocity and decrease with granule diameter. Increasing the gas velocity and particle diameter leads to increased filtration efficiency and decreased deposition uniformity. The effect of Stokes number on filtration efficiency and deposition uniformity can be divided into three regimes, and filter height and granule diameter need to be optimized according to Stokes number. A correlation of initial filtration efficiency is obtained for use in filter design and structural optimization.

A real-time filtration model is developed in this paper, and numerical simulations are presented for filtration performance of a randomly packed granular filter. Fluid flow characteristics and filtration performance at the initial state are first investigated. Correlations of initial filtration efficiency and initial pressure drop are obtained with good prediction accuracy. Particle filtration characteristics on the granule surface show that the real-time filtration model can well predict particle deposition and accumulation characteristics on the granule surface. The particle deposition fraction in the filter decreases with filter depth, and for longer filtration time, inhomogeneity appears in the axial deposition fraction. Correlations of filtration efficiency and pressure drop versus time are presented and both are of sufficient accuracy for engineering purposes. The expression of the cleaning time is obtained, which can be used to determine the optimized cleaning time for economic operation of granular filters.

An on line experimental system was set up in a heating boiler so as to reproduce dusty working environments in heat recovery systems and to investigate heat transfer and fouling performance of different kinds of finned tube heat exchangers. Measurements are reported for the heat transfer coefficients, fouling and the effect of fouling on heat transfer performance of five different H-type finned tube heat exchangers. Fouling resistance and the weakened degree of heat transfer coefficient are employed to evaluate the effect of fouling on the heat transfer performance. Numerical simulations are also conducted on the fouling performance of each heat exchanger bundle and compared to the measurements. Double H-type fins can slightly reduce the heat transfer performance but also reduce fouling. Elliptical finned tubes with the same tube pitch decrease heat transfer performance and increase fouling, and with the same relative tube pitch, significantly increase the heat transfer performance before fouling, and can effectively reduce fouling.

Granular filter is a promising method in hot gas clean-up for advanced coal gasification technologies. Numerical simulations were performed in this paper for the particle filtration characteristics of granular filters with two typical packings: body centered cubic (BCC) and face centered cubic (FCC) packings. Fluid flow characteristics were investigated and pressure drop correlations for the two granular packings were obtained. The pressure drop of both packings increases linearly with the increase of velocity, and the pressure drop of the FCC packing is significantly higher than that of BCC. The discrete phase model (DPM) was employed to trace fly ash particle motion during the filtration process, and particle deposition characteristics on the granular surface, including the particle penetration, deposition distribution of different particle sizes, and deposition distribution along filter layers were determined. The deposition fraction of particles on each layer of granules decreases with the increase of row number, and the maximum deposition location is different for BCC and FCC packings. Large particles mainly deposit on the first four layers of granules, while small particles deposit over all layers. The collection efficiency of BCC and FCC packings increases with the increase of velocity, and they decrease first and then increase with particle diameter. Correlations of collection efficiency for the two granular packings are presented and they have good prediction accuracy.

Frost melting on a vertical surface is divided into three stages: absorption, accumulation and draining, and the effect of surface wettability on melting is investigated analytically. The ratio of the volume flux of water to the melting rate is the key factor that determines meltwater motion. In the absorption stage, the ratio is greater than unity with meltwater is absorbed into the frost layer by capillary force. The volume flux of water depends on porosity and permeability of the frost layer, which are affected by the surface wettability. When the frost layer is saturated, meltwater accumulates on the surface, and the retention water is related to surface wettability. In the draining stage, meltwater flows along the surface, and the draining velocity depends on boundary conditions at the interfaces. Draining velocity increases on hydrophobic surfaces compared to that on hydrophilic surfaces owning to the presence of a slip velocity.

Thermodynamic analysis and optimization studies are performed on three transcritical nitrous oxide refrigeration cycles including a single-stage compression (SSC) cycle, a double-stage compression (DSC) cycle and a double-stage compression with an internal heat exchanger (DSCI) cycle. The thermodynamic performance is compared with that of similar cycles using CO2. The results show that the N2O SSC and DSC cycles exhibit a larger optimum COP and a lower discharge pressure compared with the CO2 cycles at given conditions. For the DSC cycle, there exists an optimum COP at certain gas cooler high pressure and intermediate pressure for both working fluids. The double-stage compression and the internal heat exchange have a significant effect on the optimum COP enhancement for N2O transcritical refrigeration cycles. Further, the introduction of the internal heat exchange can also reduce the optimum pressure for the N2O DSC cycle. The comparison between the three cycles shows that the N2O DSCI cycle has the highest optimum COP. Correlations of the optimum gas cooler high pressure and the intermediate pressure are proposed in terms of the gas cooler exit temperature and the evaporating temperature for the three cycles.

The effects of the tube wall on the fluid flow, permeability and streaming potential are reported for a porous transducer for use in a liquid circular angular accelerometer. Fluid flow and pressure near the surface of the transducer are modeled and validated numerically. We show that the differential pressure across the transducer is not affected by the tube wall. A capillary bundle model is employed to represent the transducer to obtain the radial porosity, permeability and streaming potential distributions due to wall effects. A simulated spherical packing is generated from measured particle parameters to specify the model by calculating the radial porosity distribution. The theoretical permeability and streaming potential coupling coefficient are located within one standard deviation from the mean of measurements. Due to the radial distribution of streaming potential, the electrode can be configured into a large streaming potential region to measure the signal more efficiently. Three electrode configurations are described based on these results, which can be applied to increase the signal of the sensor.

Experiments of defrost processes are reported for superhydrophilic, plain and superhydrophobic surfaces which are vertically placed. On the superhydrophobic surface, the frost layer falls off as a rigid body. On the superhydrophilic and plain surfaces, the frost melts, and part of the frost layer falls off with the draining meltwater. Defrost time is thus less for the superhydrophobic surface compared to that for superhydrophilic and plain surfaces. Frost slumping conditions are analyzed with a static force balance, and criteria for frost release are presented. Meltwater motions are suggested as the key factor of the defrost mechanism. When the volume flux of meltwater in the frost is greater than the melting rate, the meltwater is absorbed into the frost. When the volume flux of meltwater is less than the melting rate, it accumulates and drains on the surface. Water accumulation favors frost slumping because the adhesive force becomes weak. Frost slumping generally shortens defrost time and improves defrost efficiency based on our measurements.

A capillary bundle model is used to evaluate the permeability and streaming potential coupling coefficient of a porous transducer, which is the key element of a fluid-based angular accelerometer. The capillary bundle is specified with structural parameters and a method to transform the particle size distribution into the capillary radius distribution. Together with a theoretical model of zeta potential and specific surface conductance, the model is applied to determine permeability and streaming potential prediction. Three types of transducers are fabricated and experiments validate the proposed model, which is an important part of the sensor model for engineering application.

We report measurements of boiling heat transfer in a dilute emulsion in the presence of a surfactant on an upward facing flat surface. The emulsion comprises FC 72 droplets in water with Tween 20 as the surfactant at 0, 10, 60 and 100 ppm by volume. Boiling data for the emulsion without surfactant confirms previously observed trends, with an increase in heat transfer coefficient occurring near 80 °C. Boiling with the surfactant does not produce an enhancement of the heat transfer coefficient in both aqueous surfactant mixtures and emulsions with surfactant. Laser diffraction imaging shows that emulsions with different volume fractions of the disperse component have different particle size distributions from the same emulsification process. Mixtures with the surfactant tend to have a tighter particle size distribution with a mean diameter of 2 μm. The similarity of boiling behavior for mixtures of water with Tween 20 and the emulsion with surfactant suggests that the boiling behavior is characterized mostly by interface chemistry unique to the surfactant than to the effect of droplet size distribution.

Measurements of heat transfer coefficients are presented for flow boiling of a dilute emulsion in meso-scale channel flow. The emulsions comprises droplets of FC-72 and pentane in water at a droplet concentration of one percent or less by volume. Droplet size is on the order of 5–10 μm. The experimental apparatus comprises a bottom heated flat channel with a gap height of 0.25 mm and constant wall heat flux of 10–50 W/cm 2 . Mass flux is held constant at ∼100 kg/m 2 s. Heat transfer coefficients are enhanced on the order of 20–60% when the wall temperature exceeds the saturation temperature of the dilute component but is less than the saturation temperature of the carrier fluid. This increase is attributed to agitation due to the liquid-vapor transition of the suspended component. As the wall temperature approaches the saturation temperature of the carrier fluid, boiling of it commences, and heat transfer coefficients increase dramatically as in single-component flow boiling. Thus, the emulsion exhibits more desirable heat transfer characteristics across the spectrum from single-phase through two-phase flow. At very low heat flux, there is little to no differentiation of heat transfer coefficients of carrier fluid alone and that of the emulsion because the droplets require a degree of superheat before evaporating. The present results provide a useful experiential framework for categorizing flow boiling conditions associated with dilute emulsions and complement existing data on pool boiling.

Mixed convection in fluid superposed porous layers heated locally from below is analyzed numerically based on the one-domain formulation of the conservation equations for laminar flow in the fluid sublayer and Darcy-Brinkman-Forcheimer (DBF) flow in the porous sublayer. Porosity is used as the switching parameter causing the governing equations to transition from an extended form of the DBF equations in the porous sublayer to the Navier-Stokes equations in the fluid sublayer. The conductivity ratio, Darcy number, porous layer height fraction, Rayleigh number, and Peclet number have a strong effect on the Nusselt number. Complex thermal pluming above the heated zone on the lower boundary appears for certain combinations of Rayleigh and Peclet numbers. Results extend the parameter space of the natural convection case through the mixed convection regime and into the forced convection regime for fluid superposed porous layers.

Heat transfer coefficients are reported for mixed convection in a 10 cm × 10 cm horizontal channel partially filled with a porous layer of 3 mm DIA glass beads and bottom heated over a 10 cm length. Measurements were made for Rayleigh-Darcy numbers of 35–655, Peclet numbers of 0–316, and several porous sublayer heights (0, 5, 7.5, 9 and 10 cm). Heat transfer coefficients are favorably compared to prior numerical results and thus validate predictions. The Nusselt number is sensitive to the fraction of the overall layer occupied by the porous sublayer at sublayer fractions less than 0.5, but is less so at larger sublayer fractions. The numerical prediction of a critical Peclet number at which the Nusselt number is minimum is confirmed experimentally, showing that a small cross flow can reduce heat transfer by disrupting natural convection currents. The transition to forced convection occurs at larger Peclet numbers than have been predicted numerically.

Asymmetric liquid–liquid droplet heating mechanisms differ from the more commonly studied and better understood symmetric liquid–gas mechanisms. In this work, we simulate two-dimensional low Weber number droplet heating in developing low Reynolds number liquid boundary layers. Of particular interest are the influences of Weber, Prandtl, and Reynolds number magnitudes on the system evolution. We perform simulations with a coupled Eulerian–Lagrangian interface capturing methodology – the Lagrangian volume of fluid – alongside an Eulerian solver for the Navier–Stokes equations that provides the spatial and temporal evolution of the temperature and velocity fields for the droplet and the surrounding fluid. Our results show droplet rolling induced by the velocity boundary layer modifies the temperature field in and around the droplet. Conduction negates the thermal influence of rolling in low Prandtl number droplets, but modifies the continuous phase temperature field. The Magnus force separates the droplets from the heated surface, decreasing their heating rate. These results establish the fundamentals of asymmetric liquid–liquid droplet heating in developing boundary layers: it is necessary to include the Magnus force in physically representative near-wall droplet heating models, and resolution of near-droplet temperature gradients may be necessary in situations with temperature dependent interface processes.

Pressure drop characteristics in flow boiling in a vertical rectangular channel (2.0 mm × 60 mm × 1000 mm) are reported under uniform and non-uniform transverse wall heating. The test section has symmetrical wall power, and the effective heating area is 56 mm × 700 mm. A wide range of operating conditions is obtained by varying the inlet pressure and mass flow. Results show that the variation of the pressure drop gradient s with thermal parameters is similar under uniform and non-uniform wall heating. The pressure drop gradient decreases as the saturation pressure increases and increases significantly as the mass flow rate increases. It remains nearly constant or slightly decreases with increasing vapor quality at low mass flux, while it decreases slightly first and then rises rapidly at high mass flux. The homogeneous flow model using the equivalent viscosity assumption significantly underestimates experimental values, and the data dispersion is relatively large. The Beattie-Whalley correlation has a relatively good prediction, with a weighted mean absolute percent error (WMAPE) of 24.6%. The Muller-Steinhagen-Heck and Li-Wu correlations are found to be the best with WMAPE of 11.1% and 12.2% respectively, and most of the predicted values are within a ±20% error band. A new correlation based on the forms of Muller-Steinhagen-Heck correlation is proposed, the Bond number is introduced, and measurements are within ±8% of predictions. The lateral power distribution has an important influence on the flow boiling pressure drop, which can be attributed to the effects of void distribution and transverse flow. The non-uniform wall power distributions increase the pressure drop gradient compared to that of the uniform distribution.

Characteristics of single-phase convective heat transfer in a rectangular channel are reported for heated and non-heated conditions. Results indicated that the friction factors were basically the same in the laminar region. However, in the transition and quasi-turbulent region, the heated friction factor was slightly lower than the non-heated values, and the value of non-uniform heating was slightly higher than that of uniform heating. The transition started critical Reynolds number increased from 2700 without heating to about 3000 with heating only due to the viscosity decreased with increasing temperatures. Due to the transition started and ended Reynolds number increased simultaneously under heating, the width of flow transition was almost the same as that without heating. The transverse power distribution influenced the average Nusselt number of the channel, although measured differences were small for uniform and non-uniform heating conditions. For the heat transfer data under uniform heating conditions obtained in this study, 15 correlations were evaluated. It is found that the existing correlations underestimated the experimental data for Re 10000, with almost all data being within ±20% errors.

We report measurements of steady state boiling heat transfer in a dilute emulsion of FC72™ in water on upward and downward facing surfaces. Augmented heat transfer coefficients are expressed as a free convection regime followed by a significant increase when the surface temperature exceeds the saturation temperature of the disperse component. Augmented heat transfer coefficients reach three times that in free convection for all volume fractions at surface temperatures near the saturation temperature of the continuous component, followed by a rapid decrease as film boiling commences. For the downward facing surface, the augmentation effect is seen when the surface temperature approaches the saturation temperature of the continuous component. At that point, the presence of the disperse component produces a doubling of the heat transfer coefficient.

A model of frost melting is developed from fundamental analysis of the heat and mass transfer. The melting model envisions three stages as the frosted surface is heated: absorption of the melt water by diffusion, accumulation of the melt water, and draining of the melt water along the surface. The three stages of melting are connected physically, but the analysis of each involves a particular set of assumptions and use of a wide range parameters and physical laws. Surface wettability enters the analysis as a factor in the draining stage. The possibility of frost slumping is expressed through a criterion based on a static force balance in which contact angel is an implicit factor.

Flow boiling in an onboard variable microgap is demonstrated as a viable cooling method for multidevice electronics. The microgap is created by a bonded conformal encapsulation that delivers uniform subcooled inlet coolant flow across a multidevice layout comprising a processor and two in-line, symmetrically placed memory devices. Each device is simulated with a ceramic resistance heater on a 1:1 scale, and the heights of the devices create the variable microgap under the roof line of the encapsulation. The gap height for the processor is 0.5 mm and 1 mm for the memory devices. Parameters investigated are pressure drop, average device temperature, processor power, and coefficient of performance (COP). For inlet coolant flow first over the memory devices, the average device temperature exceeds the 95 °C limit when processor power is ∼50 W or less. For inlet flow over the processor, memory device temperatures are approximately the same over all the levels of processor and memory chip power. For processor power 40 W, two-phase heat transfer dominates, and processor power of 120 W is reached within the 95 °C threshold. Volumetric power density across the data set is 134 to 1209 W/cm3.

Comparative energy and exergy investigations are reported for a transcritical N2O refrigeration cycle with a throttling valve or with an expander when the gas cooler exit temperature varies from 30 to 55 °C and the evaporating temperature varies from −40 to 10 °C. The system performance is also compared with that of similar cycles using CO2. Results show that the N2O expander cycle exhibits a larger maximum cooling coefficient of performance (COP) and lower optimum discharge pressure than that of the CO2 expander cycle and N2O throttling valve cycle. It is found that in the N2O throttling valve cycle, the irreversibility of the throttling valve is maximum and the exergy losses of the gas cooler and compressor are ordered second and third, respectively. In the N2O expander cycle, the largest exergy loss occurs in the gas cooler, followed by the compressor and the expander. Compared with the CO2 expander cycle and N2O throttling valve cycle, the N2O expander cycle has the smallest component-specific exergy loss and the highest exergy efficiency at the same operating conditions and at the optimum discharge pressure. It is also proven that the maximum COP and the maximum exergy efficiency cannot be obtained at the same time for the investigated cycles.

The chapter presents an exposition of the governing equations for heat transfer between the solid and fluid phases of a saturated porous medium. Volume-averaged equations are developed from first principles based on Wittaker’s formulation for the solid and fluid phases with application to the current case where there is convective motion and dispersion in the fluid phase. A one-equation model is developed. Determining thermal dispersion is a remaining unsolved problem.

An experimental apparatus and methodology are described for investigation of heat transfer from a vertical flat plate embedded in a saturated porous medium. The goals are to develop a Nusselt number correlation for steady heat transfer and to quantify the effects of thermal dispersion in the wall region. It has been hypothesized that the wall temperature variation as a function of time and space is an indication of the flow field developing in the porous medium and therefore a noninvasive measurement technique is used. Transient temperatures provide data for examining the behavior of a porous medium during the conduction regime and the departure from it.

Onboard liquid cooling of electronic devices is demonstrated with liquid delivered externally to the point of heat removal through a conformal encapsulation. The encapsulation creates a flat microgap above the integrated circuit (IC) and delivers a uniform inlet coolant flow over the device. The coolant is Novec™ 7200, and the electronics are simulated with a resistance heater on a 1:1 scale. Thermal performance is demonstrated at power densities of ∼1 kW/cm3 in the microgap. Parameters investigated are pressure drop, average device temperature, heat transfer coefficient, and coefficient of performance (COP). Nusselt numbers for gap sizes of 0.25, 0.5, and 0.75 mm are reduced to a dimensionless correlation. With low coolant inlet subcooling, two-phase heat transfer is seen at all mass flows. Device temperatures reach 95 °C for power dissipation of 50–80 W (0.67–1.08 kW/cm3) depending on coolant flow for a gap of 0.5 mm. Coefficients of performance of ∼100 to 70,000 are determined via measured pressure drop and demonstrate a low pumping penalty at the device level within the range of power and coolant flow considered. The encapsulation with microgap flow boiling provides a means for use of higher power central processing unit and graphics processing unit devices and thereby enables higher computing performance, for example, in embedded airborne computers.

A small scale solar tracking concentrated photovoltaic–thermal (CPV–T) system was investigated to enhance the energy efficiency of photovoltaic systems. The experimental measurement was firstly done to obtain the environmental parameters and on site efficiencies of the system. A novel Non-Dimensional Lattice Boltzmann Method (NDLBM) was then developed to simulate the transient fluid flow and heat transfer of the CPV–T receiver. This NDLBM establishes a whole set of dimensionless form of lattice Boltzmann equations and boundary conditions with dimensionless governing parameters in both macroscopic and mesoscopic length scales. The relaxation time is expressed in form of the mesoscopic Reynolds number instead of the viscosity, making the relationship between the mesh size and simulation range clearer. The present physics-based dimensionless inlet/outlet flow and heat flux boundary conditions make it possible to simulate the present high solar irradiance, large temperature difference, and high velocity mixed convection heat transfer problem. The effects of the flow rate, inlet flow temperature and distributions of the inlet/outlet on the heat transfer are obtained with NDLBM simulations over a wide range of Reynolds number, Rayleigh number, and Richardson number. The results provide a full understanding of the mechanics of the efficiency enhancement of the CPV–T system due to water cooling.