Dominant Heat Transfer Mechanism Research Articles

Introducing a new concept for enhanced micro-energy harvesting of thermal fluctuations through the Marangoni effect

We present a new concept to strongly enhance the conversion of ambient thermal fluctuations into electric power using the Marangoni effect. The physics and capability of the concept are demonstrated through simulations of a micro-energy harvester formed by a Thermoelectric Generator (TEG) coupled to a Heat Storage Unit (HSU) filled by a Phase Change Material (PCM) with a free surface. Gradients of surface tension in the PCM liquid phase originate thermocapillary flows that accelerate the heat transfer between TEG and HSU. This concept can be applied to conditions where the PCM has a free surface and is particularly advantageous in spacecraft, where microgravity makes Marangoni flows a dominant heat transfer mechanism. We describe the system layout and apply the concept to micro-energy harvesting under the thermal load experienced in a small satellite orbiting the Earth. Most of the augmented energy production with respect to conductive transport occurs during the PCM heating phase when Marangoni flows are strongest. For large enough HSU to sustain continuous phase change during the harvesting period, the Marangoni effect multiplies by 2.9, 5.6, and 7.3 times the energy harvested from the environment using octadecane, hexadecane, and water as PCMs, respectively. Thermocapillary flows arise naturally at the interface of liquids subjected to thermal gradients, and no additional physical elements that would add weight or complexity are required to exploit this concept. Since it is based on a fundamental principle, it can be used as a foundation for further improvements to optimize the output of thermoelectric devices to power low-consumption electronics.

Phase change heat transfer induced by plasmon heat generation in liquid micro-layer inside a micro-reactor

This study explores the concept of utilising visible light spectrum for inducing phase change via localised surface plasmon resonance (vis-LSPR) heating in a 1 µm × 50 nm (W × H) liquid micro-layer inside a micro-reactor. Several computational physics and computational fluid dynamic models were developed, coupled, and solved to calculate the spatial–temporal electromagnetic field, surface and fluid temperatures, vapour fraction, and velocity distributions inside a micro-reactor. The micro-reactor was mediated with silver nanoparticles (AgNP) coated on titanium oxide (TiO2) substrate, to create a Schottky junction aiming at maximising vis-LSPR heating. The results of the modelling showed that the transient heat transfer is highly wavelength-dependent with the highest temperature elevation obtained at 680 nm. Phase change phenomenon was observed as the general mechanism of heat transfer that can be controlled with light wavelength. Also, the exposure time controls the intensification of the phase change and the temperature of the nanoparticles. For an exposure time of 800 µs (and wavelength of 680 nm), the highest temperature achieved is 2077 K. It was also identified that longer exposure times induce a dominant heat transfer mechanism controlled by phase change heat transfer, which is manifested as a sudden and significant increase in the temperature of the working fluid. For nanoparticle radius range from 1 to 10 nm, it was shown that increasing the particle size, the electromagnetic loss in the AgNP/TiO2 also increases almost exponentially, which in turn promoted the phase change phenomenon as well. Overall, to maximise the thermal energy release using vis-LSPR, the key operating parameters, namely the size of the nanoparticle, wavelength, and exposure time, can be tuned to achieve the required fluid temperature for the target application. Also, the phase change phenomenon was the main mechanism of heat transfer accounting for fast heat transfer from the plasmonic layer to the liquid bulk.

Effect of copper core diameter on heat transfer and horizontal flame spread behaviors over electrical wire

The laboratory polyethylene wire with different core diameters of 3 mm, 5 mm, 6 mm, 7 mm, 8 mm, 10 mm and 12 mm was used to study the flame spread characteristics. The flame spread could be divided into initial oscillation stage, stable oscillation stage and burn-out stage. With the increase of the copper core diameter, the flame width and flame area increased firstly and then decreased. For wires with intermediate copper core diameters of 6 mm, 7 mm, 8 mm and 10 mm, the relationship between flame area and core diameter was established as A~dc, the total mass loss rate k with the copper core diameter dc followed as k~dc, and the combustion mass loss rate and dripping mass loss rate gradually equaled to each other at stable oscillation stage. By analyzing the temperature field during the process of flame spread, the temperature gradient was almost same for diameters from 3 mm to 10 mm, which demonstrated the radiation heat would be influenced by flame height. Combined with the increased flame spread, the dominant conduction heat transfer mechanism was built. When the diameter was 12 mm, the smaller conduction heat transfer, which illustrated the core material Cu played an cool effect on flame, thus the dominant heat transfer of Cu was changed from a heat source to a heat sink.

Analysis on Two-phase Heat Transfer Characteristics in Horizontal Mini-tubes

The two-phase heat transfer characteristics of degassed water flow boiling in horizontal mini-tubes are experimentally investigated in this paper. The inner diameters (di) of the tubes are 1.2mm, 1.6mm, and 2.0mm. Experiments are conducted with heat fluxes between 1.5kW/m2 and 155.75kW/m2 at mass fluxes of 21.19, 42.39, and 84.77kg/(m2s). The boiling curves of the three tubes are obtained. Incipient hysteresis is observed only in the tube with di = 1.2mm. Two-phase dominant heat transfer mechanisms are analyzed in the tubes with di = 1.2mm and di = 2.0mm based on the curves of local heat transfer coefficient versus vapor quality and the visualization results. The trend of peak value of the local heat transfer coefficient versus heat flux at different mass fluxes in the tube with di = 1.2mm is discussed in accordance with the thickness variation of the liquid film in annular flow.

Dominant Heat Transfer Mechanisms in the GTAW Plasma Arc Column

Dominant Heat Transfer Mechanisms in the GTAW Plasma Arc Column

Heat transfer, pressure drop and flow patterns of flow boiling on heterogeneous wetting surface in a vertical narrow microchannel

The micro-structured surfaces have a significant impact on the flow patterns and heat transfer mechanisms during the flow boiling process. As illustrated by previous studies, there is a trade-off between a hydrophobic and a hydrophilic surface. To examine the effect of heterogeneous wetting surfaces on the flow boiling process, an experimental investigation of flow boiling in a rectangular vertical narrow microchannel with the heterogeneous wetting surfaces and original silicon surface was conducted with deionized water as the working fluid. Hydrophobic patterns perpendicular (HC) and parallel (HP) to the flow direction are compared. The subcooled boiling curves, heat transfer coefficient, and pressure drop were discussed with the variation of heat flux and mass flux, the trends of which were analyzed along with the flow patterns. The heat transfer coefficient of flow boiling on heterogeneous wetting surface HC was enhanced for up to 39.55% compared to a silicon surface with less pressure drop. During the boiling process, the dominant heat transfer mechanism was nucleate boiling, with numerous nucleate sites between the hydrophilic/hydrophobic stripes. The easily formed bubble can be easily detached due to the supply of liquid from the hydrophilic regions nearby. With the increasing wall heat flux, the boiling heat transfer coefficient of the heterogeneous wetting surface HP is gradually lower than HC due to hydrophobic stripes parallel to the flow direction cannot restrict the bubbles from expanding in the axial direction.

The effect of surface wettability on flow boiling characteristics within microchannels

The process of flow boiling within micro-passages plays a very important role in many industrial applications. However, there is still a lack of understanding of the effect of an important controlling parameter: surface wettability. In this paper, an advanced numerical investigation on the effect of wettability characteristics on single and multiple bubble growth during saturated flow boiling conditions within a microchannel is performed. The 3D numerical simulations are conducted with the open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM, utilising a custom user-enhanced Volume Of Fluid (VOF) solver. The proposed solver enhancements involve an appropriate treatment for spurious velocities dampening, an improved dynamic contact angle treatment, as well as the implementation of a phase-change model in the fluid domain also accounting for Conjugate Heat Transfer (CHT) with the solid domain. In total, three sets of simulations of hydrophilic and hydrophobic surfaces with constant heat and mass flux were performed. In the first set, a single bubble seed was patched close to the inlet of the microchannel and the Heat Transfer Coefficient (HTC) along the channel interface was measured until the nose of the bubble reaches the outlet. The bubble growth and transport process within the channel were analysed, with a minor effect of the wettability characteristics on the HTC observed. In the second set of simulations, multiple recurring nucleation events at the same position were simulated; observing that in such more realistic cases the effect of wettability in the HTC was more profound. Finally, simulations with multiple nucleation sites and recurring nucleation events were conducted to analyse cases closer to reality. These results show indeed that surface wettability plays a significant role on the HTC, with the hydrophilic and hydrophobic cases performing approximately 43.9% and 17.8% higher respectively, compared to the single-phase reference simulations. Additionally, it is found that the dominant heat transfer mechanisms for the hydrophilic and hydrophobic surface are liquid film evaporation and contact line evaporation, respectively, and that for the proposed simulation parameters liquid film evaporation can be considered as a more efficient heat transfer mechanism compared to contact line evaporation.

Experimental Investigation Into the Heat Transfer Mechanism of Oscillating Heat Pipes Using Temperature Sensitive Paint

Abstract Oscillating heat pipes (OHPs) represent a promising passive mechanism for the removal or spreading of heat. While simple to construct, the fluid and thermodynamics of these devices are still poorly understood. There is debate over whether the primary heat transfer mechanism is due to sensible heating of the liquid phase or due to latent heat transfer through phase change. To answer this question, an experimental apparatus was constructed to provide time- and space-resolved temperature and heat transfer data across the face of an operating OHP with HFE-7000 as the working fluid. This experiment utilized temperature sensitive paint (TSP) alongside visual recording of the fluid motion in order to determine the relative latent and sensible contribution to the overall heat transfer. The OHP was tested with input powers ranging from 2.6 W to 10.1 W. It found that latent heat transfer was the dominant heat transfer mechanism, accounting for between 65% and 83% of the total heat transferred in all cases.

Dominant heating mechanisms in a surface barrier discharge

In computational models of atmospheric pressure surface barrier discharges (SBDs) the role of heating of the dielectric material and the quiescent gas is often neglected, impacting the accuracy of the calculated chemical kinetics. In this contribution, a two-dimensional fluid model of an SBD was developed and experimentally validated to determine the relative contribution of the dominant heat transfer mechanisms and to quantify the impact of discharge heating on the resultant chemistry. Three heating mechanisms were examined, including electron heating of the background gas due to inelastic collisions, ion bombardment of the dielectric surface and dielectric heating by the time-varying electric field. It was shown that electron heating of the background gas was not significant enough to account for the experimentally observed increase in temperature of the dielectric material, despite being the dominant heating mechanism of the gas close to the electrode. Dielectric heating was ruled out as the frequency response of typical dielectric materials used in SBD devices does not overlap with the experimentally observed power spectrum of an SBD excited at kHz frequencies. The ionic flux heating was found to be the dominant heating mechanism of the dielectric material and the downstream flow driven by the SBD. The largest impact of plasma heating on discharge chemistry was found in reactive nitrogen species (RNS) production, where the densities of RNSs increased when an appropriate treatment of heating was adopted. This had a marked effect on the discharge chemistry, with the concentration of NO2 increasing by almost 50% compared to the idealized constant temperature case.

Thermo-magneto-convection and mechanism of multiple eddy formation due to applied magnetic field in a lid-driven cavity

Magnetohydrodynamic study of eddy formation and heat transfer in a bottom heated two-dimensional lid-driven cavity with the insulated sidewall is reported in this paper. We consider the classic problem of a two-dimensional lid driven cavity in the presence of an applied external magnetic field. In the standard nonlinear Navier–Stokes equation and the energy equation, appropriate terms are added to model the applied magnetic field and thermo-magnetic convection. And we have derived a higher-order numerical scheme that can provide solutions that are fourth-order accurate. The scheme is implemented with suitable fourth-order accurate boundary conditions. The square LDC enclosure is filled with a fluid of Pr = 6.2. The top lid of the cavity is kept moving with constant velocity. The effect of magnetic field on heat transfer and eddy formation is discussed for 0 ≤ Ha ≤ 250, and 103 ≤ Gr ≤ 104 and the results are presented in terms of contour plots of streamlines, isotherms, Lorentz force, and the kinetic energy. The physical mechanism for forming multiple recirculation zones is understood and attributed to two opposite forces acting in the fluid about the center-line of the cavity, which acts like a couple (and hence rotational flow). This is discovered by computing the Lorentz force acting in the entire domain of fluid flow. Similarly, from the computation of kinetic energy of the fluid at all the grid points in the fluid region, it is revealed that conduction is the dominant heat transfer mechanism at higher magnetic fields. At the same time, convection is the prime mode at low magnetic fields. It is ascertained that with an increase in magnetic field intensity, velocity profiles are suppressed, and heat transfer stabilizes, giving rise to the formation of recirculation zones in the cavity.

Theoretical study of heat transfer enhancement mechanism of high alcohol surfactant in spray cooling

High alcohol surfactant could enhance spray cooling heat transfer, however the enhancing mechanism had not been identified. The present work used numerical model of spray cooling for high alcohol surfactant to specify the mechanism. After adding high alcohol surfactant, the spray characteristics and fluid properties were changed. The influences of these factors on heat transfer were studied, and it was found that the enhancement of heat transfer was mainly attributed to the effect of high alcohol surfactant on surface tension and droplet diameter: the decrease of surface tension and increase of droplet diameter made liquid film thicker and faster, and the lager liquid film thickness and velocity were helpful to take away the heat with a better flow. Both parameters were included in Weber number of droplets, and it could be concluded that high alcohol surfactant improved spray cooling performance mainly because of the increased Weber number. The dominant heat transfer mechanism was also discussed in the analysis of the effect of spray flow.

Numerical Research to Determine the Dominant Mechanism of Mass and Heat Transfer in Pressure Swing Adsorption Processes

Numerical Research to Determine the Dominant Mechanism of Mass and Heat Transfer in Pressure Swing Adsorption Processes

Water Temperature Controls for Regulated Canyon‐Bound Rivers

AbstractMany canyon‐bound rivers have been dammed and downstream flow and water temperatures modified. In some regions, climate change is expected to cause lower storage in reservoirs and warmer release temperatures, which may further alter downstream flow and thermal regimes. To anticipate potential future changes, we first need to understand the dominant heat transfer mechanisms in canyon‐bound river systems. Toward this end, we adapt a dynamic process‐based river routing and temperature model to account for complex shading and radiation characteristics found in canyon‐bound rivers. We apply the model to a 362 km segment of the Colorado River in Grand Canyon National Park, USA to simulate temperature over an 18‐year period. Extensive temperature and flow data sets from within the canyon were used to assess model performance. At the most downstream gaging location, root mean square errors of hourly flow routing and temperature predictions were 11.5 m3/s and 0.93°C, respectively. We found that heat fluxes controlling temperatures were highly variable over space and time, primarily due to shortwave radiation dynamics and hydropeaking flow conditions. Additionally, the large differences between air and water temperature during summer periods resulted in high sensible and latent heat fluxes. Sensitivity analyses indicate that reservoir release temperatures are most influential above the RM88 gage (141 km below Glen Canyon Dam), while a combination of discharge, shortwave radiation, and air temperature become more important farther downstream. This study illustrates the importance of understanding the spatial and temporal variability of topographic shading when predicting water temperatures in canyon‐bound rivers.

Convective heat transfer of nano-encapsulated phase change material suspension in a divergent minichannel heatsink

The convection heat transfer of nano-encapsulated phase change material (NEPCM)-water suspensions in a divergent minichannel heatsink was experimentally investigated. The mini heatsink was made of eight minichannels with a divergent angle of 1.38°. The minichannel was heated at the bottom, and the NEPCM-water was used as the cooling working fluid. The average Nusselt number, the index of performance, and the pressure drop of NEPCM-water compared to the pure water were examined. The experiments show that employing NEPCMs could be advantageous in low Reynolds number flows and low heating powers. The phase change heat transfer of NEPCM particles promoted the heat transfer by 82% compared to pure water. Moreover, using the NEPCM particles was not advantageous at large Reynolds numbers, where the sensible heat was the dominant mechanism of heat transfer.

Thin liquid films formation and evaporation mechanisms around elongated bubbles in rectangular cross-section microchannels

Thin film evaporation is the dominant mechanism of heat transfer in the microchannel flow boiling process. Hence, its understanding is paramount to development of predictive models and more efficient microchannel heat exchangers. Although microchannels mostly have a rectangular cross-section, the existing models are developed for circular cross-section microchannels. The non-uniformity of the liquid film thickness in microchannels with a rectangular cross-section has made prediction of the onset of dryout and heat transfer coefficient greatly challenging. Here, a test device capable of measuring the wall heat flux is utilized to fully characterize the thin film formation and evaporation process. The effects of flow capillary number (Ca) and channel aspect ratio on the liquid film thickness are determined in microchannels with 300 μm width and 300, 150, and 75 μm heights, representing aspect ratios of 1, 2, and 4, respectively. It is shown that the only existing mechanistic model for the elongated bubble regime in microchannels fails to predict the experimental heat transfer coefficient (HTC) both quantitively, by a 50–120% margin, and qualitatively, due to a fundamental difference in evaporation and dryout of a variable versus a uniform film assumed in the model. The results presented here pave the way for development of a more accurate mechanistic model for the thin film evaporation heat transfer mechanism in microchannels with a rectangular cross-section.

Numerical analysis and improvement of the thermal performance in a latent heat thermal energy storage device with spiderweb-like fins

• A bionic spiderweb-like fin is designed and optimized for latent heat storage. • A solidification model with consideration of free convection is developed. • Free convection is conducive to the overall energy discharging performance. • Spiderweb-like fin with a bifurcation number of 8 maximizes thermal performance. • Solidification time reduces by 47.9% and discharging rate increases by 1.44 times. Solidification enhancement is an essential topic for sustainable energy development. An innovative latent heat thermal energy storage (LHTES) device employing spiderweb-like fins is designed to improve solidification efficiency. The numerical solidification process of the current LHTES device is performed and compared to that with plate fins occupying an identical fin volume. Moreover, the solidification strengthening mechanism is revealed as well as the role of free convection in the heat release performance. Finally, the configuration of the spiderweb-like fin is improved, and the solidification efficiency of LHTES devices is evaluated. The results imply that the LHTES device with spiderweb-like fins improves the solidification performance by eliminating the heat transfer hysteresis zone. The dominant heat transfer mechanism of the solidification process shifts from convective heat transfer in the early stages to heat conduction in the later stages. More significantly, the free convection strengthens the solidification performance of the LHTES device with plate fins but has little effect on that with spiderweb-like fins. A spiderweb-like fin with the bifurcation number of 8 is suggested for maximizing the discharging performance. Compared to the plate fin, the full solidification time of the improved LHTES device with spiderweb-like fins is shortened by 47.9%, and its corresponding time-averaged heat release rate increases by 1.44 times.

Low Frequency Pulsating Jet Impingement Boiling and Single Phase Heat Transfer

Experiments are carried out to characterize the influence of jet pulsations on the single phase and boiling heat transfer under a confined and submerged impinging jet, using water and FC-72. A novel jet pulsation mechanism is developed for introducing the mean flow pulsations at different jet flow rates. The effects of jet Reynolds number, pulsation frequency and amplitude are studied by comparison of the time-averaged Nusselt numbers and boiling curves against steady state data. The root-mean-squared dimensionless temperature oscillations in the heater are correlated to the jet Strouhal number, mean Reynolds number and the diffusion Fourier number of the heater that is based on the frequency of jet oscillations. Under both boiling and single phase cooling conditions, temperature oscillations are found to be inversely related to the Strouhal number with an exponent in the range 0.63-0.75. During pulsating jet impingement boiling, a periodic renewal of the boiling process is observed, where the bubbles on the heater surface are cyclically flushed downstream of the stagnation point in-phase with the pulsating flow. For the cases considered, the root-mean-squared dimensionless temperature is found to be negligible during fully developed nucleate boiling, indicating that the heat transfer process is pseudo-stationary. Here, vigorous ebullition is the dominant mechanism of heat transfer and suppresses any temperature oscillations that can arise due to the flow oscillations. For heat fluxes in the partial nucleate boiling regime and close to critical heat flux however, temperature oscillations decrease with an increase in jet Strouhal number. No marked influence of low frequency jet pulsations is found on the time averaged single phase or boiling heat transfer characteristics (including critical heat flux) for the range of parameters studied.

Wall heat flux partitioning during subcooled flow boiling of NaCl solutions and pure water

A careful evaluation on the wall heat flux partitioning is of great importance for the deep understanding the nucleate flow boiling heat transfer to saline solution. In this work, the portion of each heat transfer contribution was analyzed through wall heat flux partitioning model after model validation. The key parameters used in the model were discussed and the reasonable values were determined. The experimental data of bubble departure diameter, departure frequency, active nucleation site density, bubble waiting time and growth time, were combined into the model to eliminate the uncertainties of empirical closure sub-models. The comparison of heat flux partitioning of subcooled flow boiling between NaCl solution and pure water was presented under different operating conditions. The results showed that for the operating conditions of Qw > 180 kW/m2 or G < 400 kg/(m2·s), the contribution of single-phase liquid convection in the heat transfer of saline solution is greater than that in pure water, due to the bigger bubble diameter and more intensive bubble-induced turbulence. Quenching heat flux for NaCl solution is lower than that in pure water, owing to the smaller bubble departure frequency. For the lower heat flux or higher mass flux conditions, however, the contrary trend was observed. The dominant heat transfer mechanisms for solution and pure water during subcooled nucleate flow boiling were revealed. The interactions between bubble dynamic and heat transfer characteristics were carefully investigated.

Sulfur heat transfer behavior for uniform and non-uniform thermal charging of horizontally-oriented isochoric thermal energy storage systems

Elemental sulfur is a low-cost, chemically stable thermal storage medium suitable for many medium to high temperature applications. In this study, we investigate the heat transfer behavior of sulfur, isochorically stored in a horizontally-oriented thermal storage element (steel tube) using experimental, analytical, and computational methods. The sulfur container was uniformly and non-uniformly heated along its axis from 50 to 600 °C to simulate the potential operating conditions for the full-scale thermal energy storage systems. The results of the study reveal distinct sulfur heat transfer mechanisms based on the temperature range and mode of thermal charging. For temperatures from 50 to 200 °C, the sulfur heat transfer behavior is governed by two primary mechanisms; 1) solid–liquid phase change, and 2) sulfur viscosity that varies strongly with temperature. From 200 to 600 °C, the buoyancy-driven natural convection is the dominant heat transfer mechanism and facilitates significantly high thermal charge rates. For axially non-uniform thermal charging, the axial temperature gradient induces natural convection along the axis that rapidly redistributes the thermal energy within the sulfur mass. Such axial convection has a strong impact on the thermal characteristics, including thermal charge/discharge rate and exergetic efficiency of the thermal storage systems. These observations and the high-fidelity computational model used in this study provide important means to identify the design parameters and operating conditions for which sulfur-based thermal energy storage (SulfurTES) systems will provide desirable thermal performance at a low thermal storage cost.

Effect of the heat transfer liquid fin on critical heat flux enhancement under ERVC condition

Several critical heat flux (CHF) enhancement methods for in-vessel retention through external reactor vessel cooling (IVR-ERVC) have been researched. However, they have some limitations for practical applications. In the present study, heat transfer liquid fin concept was suggested to address the issue of CHF by spreading focusing effect of the heat flux. The liquid fin was located outside of the reactor pressure vessel; it is activated by filling under the accident situations. Liquid metal and oil were selected as the fin materials as they are characterized by significantly different Prandtl numbers; this property helps to clearly distinguish the heat transfer mechanisms involved. Analysis were conducted based on both experimental and numerical methods. As expected, conduction and natural circulation were the dominant heat transfer mechanisms in the case of liquid metal fin and the oil fin, respectively. Regardless of the Prandtl number of the fin, safety margin for the CHF was significantly increased in both fin cases. The heat flux profile of the conduction dominant liquid metal fin showed a moderately flattened shaped. However, for the oil fin, heat flux was mitigated and upwardly shifted (natural circulation as the primary heat transfer mechanism). Although both materials exhibited good performance in terms of securing a margin for CHF, temperature increment by the liquid fin was much higher in the oil case because of the relative lower thermal conductivity of the latter. Thus, the use of low thermal conductivity fins requires the temperature limit to be taken into consideration.