Lower Wall Temperature Research Articles

Secondary size distributions for single drop impacts at high wall superheat

The impingement of liquid sprays on hot walls is used extensively in both spray-cooling systems and in combustor fuel injection applications. At low and moderate wall temperatures, the secondary size distributions have been reported in the literature. For high wall superheat conditions, particularly for real multicomponent fuels, this secondary size distribution has received less attention. Understanding the resultant size distribution for a spray-wall impact is key to capturing vaporization and local mixture for fuel-spray impingement. In this study, single drop impacts for a range of single-component (n-decane) and multicomponent jet fuel (F-24) are characterized through dual-view imaging. Secondary droplets are captured for impact Weber numbers of 100–600 and wall temperatures spanning the nucleate and film boiling (Leidenfrost) regimes. Imaging through a transparent sapphire substrate is used to capture the impact phenomena and impact-induced breakup of impacting drops. We report empirical correlations for the secondary droplet size for single-component (n-decane) and multicomponent (F-24) liquid fuels with varying wall temperature to provide validation datasets for spray-wall simulations.

Numerical investigation on flame behaviors of NH3/H2/air mixtures in a micro-flow reactor with a controlled wall temperature profile

Flame behaviors of stoichiometric NH3/H2/air mixtures (NH3:H2 = 1:1) in a micro-flow reactor (MFR) with a controlled wall temperature distribution were numerically examined. Three flame regimes were identified at high, medium and low inlet velocities, respectively, i.e., “coexistence of an upstream normal flame and a downstream weak flame (UNF+DWF)”, “flame with repetitive extinction and ignition (FREI) accompanied by triple bifurcations”, and “single weak flame (WF)”. It was revealed that the laminar burning velocity of NH3 is increased by H2 blending, and thus the UNF of NH3/H2 blends can be stabilized farther upstream with lower wall temperature compared to that of pure NH3 flame. This leads to incomplete consumptions of H2, NO, etc., and these species react downstream of the channel with a higher wall temperature to form the DWF. Another reason for the existence of DWF is that it is located sufficiently far from the UNF. The reasons for the triple bifurcations within one cycle of FREI are as follows. A normal flame is generated through the first bifurcation, which is due to the residual H2 and NO from the incomplete combustion of both current FREI and flames of the previous period. A weak flame is formed by the second bifurcation, which is attributed to leaked H2, NO, and HNO from the FREI. After the extinguishment of FREI, large amounts of H2, NH3 and O2 diffuse downstream and the third bifurcated flame (normal flame) is formed. In conclusion, the present study discovered and interpreted the complicated flame dynamics of NH3/H2 blends in an MFR with a wall temperature gradient.

Reduction of Heating Energy Demand by Combining Infrared Heaters and Infrared Reflective Walls

We study the potential of infrared (IR) heaters in combination with IR reflective walls to reduce heating energy demand in buildings. Using IR heaters increases radiant temperature. Combined with IR reflective walls, less radiant heat is absorbed by the surrounding walls, and more is reflected to and absorbed by the occupants. This allows for lower air temperatures while maintaining constant thermal comfort. Lower air temperatures result in heating energy savings. In simulations, we examine the impact of four parameters on the thermal comfort indicator Predicted Mean Vote (PMV): wall temperature, inlet air temperature, IR heater power, and IR emissivity of the walls. To reduce the number of data points needed, we use a Central Composite Design for the layout of the simulation plan. The results show that the PMV can be changed from 0.15 to 1.16 only by lowering the emissivity of the surrounding walls from 0.9 to 0.1. At high IR heater power and at low wall temperature the impact of the emissivity on the PMV becomes larger. From the simulation data, we derive a response surface function to determine the required IR heating power for any given room conditions, which could be used for automated IR heater control.

Direct numerical simulation of mechanism and control of secondary instability induced transition in a supersonic boundary layer

Mechanisms and control of secondary-instability-induced-transition in a supersonic boundary layer are studied numerically via direct numerical simulation. The aim is to investigate and compare the transition mechanisms of fundamental, subharmonic, asymmetric subharmonic, and detuned resonances, and to control these secondary instabilities using a local wall cooling strip. The results indicate that the nonlinear interaction between the high-amplitude primary mode and low-amplitude secondary modes is the main contributor to transition. The mutual- and self-interactions of the primary and secondary modes generate other harmonic modes with laminar breakdown soon appearing. The asymmetric subharmonic resonance induces the earliest transition, while the fundamental subharmonic has the latest. Wall cooling effects are also studied. The results show that a lower wall temperature significantly suppresses the secondary instabilities, and steady modes become dominant and lead to obvious streamwise vortexes. Numerical data demonstrate that all secondary-instability-induced transitions result in fully developed turbulent boundary layers, as supported by the skin friction and scaled velocity profiles. The transition control cases indicate that the local wall cooling strip can significantly delay the transition by suppressing the growth of the primary mode. An upstream control strip is found to have a more obvious suppression effect. The fundamental and asymmetric subharmonic resonances are sensitive to the location of the local wall cooling strip and show a stronger transition delaying effect.

Numerical study of wall cooling effects on diesel spray combustion characteristics under different ambient conditions

The effects of wall cooling on diesel spray combustion characteristics and engine performance are varied under different environmental conditions and the reasons remain to be further analyzed. The systematic explanation of its influence laws can effectively help the cooling system design and fault isolation under changing working conditions. Therefore, the effect mechanism of wall temperature on the diesel spray combustion characteristics under different ambient temperature and pressure conditions is numerically revealed through the combination of CFD simulation and chemical reaction kinetics analysis. During the calculation model validation, the simulation results showed good agreement with the experimental results and good adaptability to the change in wall temperature. The results show that high ambient temperature and pressure lead to the acceleration of the reaction rate, and high pressure also slows down the fuel diffusion, which eventually causes the fuel to undergo more reaction processes before hitting the wall and being in the relatively late stage of the low-temperature reaction process. In the process of low-temperature reaction and heat accumulation, the effect of temperature drop in the late stage of the reaction is less than that in the early stage due to more heat accumulation before cooling. This ultimately leads to diesel ignition being delayed by low wall temperatures under low ambient temperature and pressure conditions, while diesel spray at high ambient temperatures and pressures has a stronger anti-interference ability to the wall temperature.

Experimental study of the influence of buoyancy on turbulent mixed convection in lead–bismuth eutectic flow under inclination conditions

The effects of inclination conditions on the heat transfer characteristics of lead–bismuth eutectic in a vertical circular tube are investigated experimentally. The results show that, compared to the vertical flow, inclination conditions result in lower wall temperature, increased fluid temperature and higher local heat transfer intensity of LBE. The effects of inclination conditions on the heat transfer characteristics of LBE flow decrease with increasing Reynolds number. Mixed convection determination criterion Y divides the heat transfer region into mixed convection and forced convection zones. The threshold value is around 2.0 × 10-3. The inclination conditions enhance the heat transfer characteristics only under mixed convection when the natural convection due to buoyancy force is more significant compared to the forced convection. The effects of inclination conditions increase with Y and inclination angle and decrease with Peclet number. The heat transfer correlation for LBE vertical flow cannot predict the Nusselt number under inclination conditions well. Therefore, a relevant empirical correlation for inclination conditions is developed. The research would be helpful for the mechanism analysis and engineering design when liquid metals are applied for motion conditions.

Characterization of very-large-scale motions in supersonic and hypersonic turbulent boundary layers

Very-large-scale motions are commonly observed in moderate- and high-Reynolds-number wall turbulence, constituting a considerable portion of the Reynolds stress and skin friction. This study aims to investigate the behaviour of these motions in high-speed and high-Reynolds-number turbulent boundary layers at varying Mach numbers. With the aid of high-precision numerical simulations, numerical experiments and theoretical analysis, it is demonstrated that the very-large-scale motions are weakened in high-Mach-number turbulence at the same friction Reynolds numbers, leading to the reduction in turbulent kinetic energy in the outer region. Conversely, the lower wall temperature enhances the very-large-scale motions but shortens the scale separation between the structures in the near-wall and outer regions.

THERMAL AND HYDRODYNAMIC ANALYSIS OF NANOFLUID FLOW IN A CIRCULAR PIPE USING EULER-GRANULAR MIXTURE MODEL

THERMAL AND HYDRODYNAMIC ANALYSIS OF NANOFLUID FLOW IN A CIRCULAR PIPE USING EULER-GRANULAR MIXTURE MODEL

Enhancing pool boiling heat transfer of modified surface by 3-D Lattice Boltzmann method

In this study, pool boiling from micro-pillar modified surface has been simulated numerically by a 3-D lattice Boltzmann method. Effects of geometries and wettability of micro-pillar on boiling heat transfer performance were also systematically evaluated. Result showed that compared within micro-pillar surface, heat flux of cubic micro-pillar surface was the highest with the lowest wall temperature. In addition, compared to hydrophilic condition, Heat flux of cubic micro-pillar surface with hydrophobic wettability increased by 98.3%. This is because hydrophobic wettability influenced nucleation site density, vapor-liquid-flow field and heat transfer performance much more than cubic shaped geometry. Finally, heat flux of cubic micro-pillar surface with hybrid wettability increased by 430.7% compared to pure hydrophilic wettability. That is due to optimal hybrid wettability surface could control nucleate site location, restrict bubble growth, and increase obviously heat transfer performance.

Investigation of Ash Deposition Characteristics in Full-Scale Circulating Fluidized Bed Boiler Burning Zhundong Coal

ABSTRACT Due to the high content of alkali and alkaline earth metals (AAEMs) in Zhundong coal, serious ash-related problems such as slagging and fouling occurred in the process of utilization, which has attracted wide attention. In this work, the ash deposition characteristics in a new type circulating fluidized bed boiler burning Zhundong coal was studied. The effect of steam cooled flue on alleviating slagging problems was evaluated, and the influence of flue gas temperature and wall temperature on ash deposition characteristics were studied. The results showed that the mass of ash deposition and the concentration of submicron particles decrease gradually along the direction of flue gas flow in the steam cooled flue. The deposited ash changed from tightly bound to wall to loosely deposited, which indicated that the arrangement of the steam cooled flue greatly alleviated the slagging problems. Compared with flue gas temperature, the wall temperature had a greater impact on ash deposition characteristics. Mineral elements such as Na, S, Fe and Mg were more likely to enrich on the high temperature wall, while the Ca showed the opposite trend. On the low temperature wall, Na existed in the form of Na2SO4 and Na2Si2O5, while on the high temperature wall, the direct condensation of alkali metal vapor was prevented and the thermophoresis disappeared, so Na mainly existed in the form of Na2Si2O5.

Optimizing spray cooling systems: A comprehensive study on influential parameters and their effects on heat transfer under varied heat flux conditions

This research explores the heat transfer characteristics of spray cooling under heat flux conditions ranging from 10 to 200 W/cm2 through numerical simulations. We examined the effects of spray height, spray pressure, surface tension, contact angle, system pressure, and gravity on spray cooling efficiency, to optimize spray cooling systems. Furthermore, we validated the simulation results through experiments and found a good agreement between the two. Key findings include a consistent increase in the heat transfer coefficient with rising heat flux. Under specific heat flux density, increased spray pressure leads to a gradual decline in wall temperature and improved wall temperature uniformity, but the reduction rate slows as pressure continues to increase. Lower spray heights can improve cooling efficiency, but may reduce temperature uniformity. Additionally, heat transfer performance in the two-phase zone improves with higher surfactant (Tween 20) concentration, with a 75ppm solution reducing cooling wall temperatures by 7.4 K compared to pure water. Spray processes at 10 kPa and 50 kPa result in lower wall temperatures before entering the two-phase zone compared to atmospheric pressure, improving cooling efficiency and temperature uniformity. Finally, differences in spray cooling effects under varying gravities manifest primarily during the two-phase heat exchange stage, with the temperature difference widening with increasing heat flux density. These insights offer valuable guidance for spray cooling applications in electronic thermal management.

Annular swirling jet reactor for converting hydrocarbons to olefins and aromatics

This paper describes a novel reactor for high temperature thermochemical conversion of ethane to olefins. The central feature is an efficient flow pattern with a large recirculation zone that provides stable and compact 2500 °C flame and a thin conical annular swirling jet that provides fast transfer of mass, momentum, and heat. The jet is directed through a unique converging–diverging nozzle that maintains relatively low wall temperature and consequently reduces the need for external cooling. The hydrocarbons stream is introduced tangentially through volutes in such a way that there is initially heat transfer with minimal mass transfer followed by fast mixing of countercurrent streams. CFD simulations with detailed kinetics were employed to design and optimize this reactor including the order of the volutes to feed ethane, H2 fuel, oxidizer (O2 and N2 mix), and a cooling N2 gas; oxygen enrichment of the air, and also the overall operating conditions of this flexible and robust reactor. This highly intensified reactor was fabricated and tested at a semi-pilot scale with up to 15 kg/hr feed. The optimized reactor has high selectivity to ethylene and acetylene comparable to conventional steam cracker.

Interactions of turbulence and flame during turbulent boundary layer premixed flame flashback under isothermal and adiabatic wall conditions using direct numerical simulation

In this work, lean hydrogen/air premixed flame flashback in a turbulent boundary layer along an isothermal wall and an adiabatic wall was simulated using direct numerical simulation. The general characteristics of flame flashback were analyzed. It was found that the propagating speed along the adiabatic wall is higher than that along the isothermal wall. The displacement speed near the isothermal wall is notably lower than that near the adiabatic wall, primarily due to the smaller value of the reaction component of the displacement speed near the isothermal wall. Furthermore, the properties of the boundary layer turbulence during flame flashback were examined in terms of the flow topology and the anisotropy of Reynolds stress. It was observed that the focal topologies tend to prevail near the adiabatic wall, and the volume fraction of stable topologies increases near the isothermal wall due to the increased compressed region caused by the wall heat loss. Finally, the influence of turbulence on the flame structure was highlighted. It was suggested that the unstable topologies dominate the heat release rate away from the wall. The proportion of heat release rate in focal topologies increases near the adiabatic wall, and the proportion in stable topologies increases near the isothermal wall. The impact of the low temperature wall on the flame structure is more pronounced in topologies with compression compared to those with expansion.

Experimental evaluation on pre-swirling cold air for flue cooling

The operation of a diesel engine leads to the emission of a substantial amount of waste heat flue gas, resulting in a continuous increase in the wall temperature of the exhaust system. To mitigate this continuous rise in temperature, a pre-swirling device has been developed to enhance the heat exchange between cold air and the wall, thus averting heating wall by the hot air. In this study, an experimental system was devised and implemented to analyze the dynamic behavior of the wall temperature in an air duct. The system considered both with and without the pre-swirling device, incorporating different cold air temperatures and flow rates to examine its impact on the increase in wall temperature. The results revealed a gradual rise in wall temperature after reaching a steady state, particularly demonstrating lower wall temperatures at 19 measurement points in the presence of pre-swirl compared to without swirl. Notably, the maximum temperature rise in the air duct wall was reduced by 55.7 %, from 7.9 °C without pre-swirl to 3.5 °C with the inclusion of the pre-swirling device, highlighting the significant thermal protection it provides and suggesting potential applications in exhaust thermal protection engineering. Moreover, the experimental findings consistently indicate that the incorporation of a pre-swirling device effectively diminishes the increase in wall temperature across different cold air flow rates and temperatures.

Experimental and simulation study on evaporation characteristics of E85 impinging spray under DISI cold-start condition

The direct injection of ethanol-gasoline blended fuel into the engine cylinder near the top dead center (TDC) is evitable to suffer from the wall impingement due to more injection mass required to maintain the same air/fuel ratio with traditional fossil fuels. In this study, the evaporation characteristics of E85 cold-start impinging spray were investigated by experiments and numerical simulation. A constant volume vessel was used to produce the direct-injection spark-ignition (DISI) engine-like conditions. The ultraviolet (UV) – visible (Vis) dual-wavelength laser absorption scattering (LAS) technique was employed to quantitatively measure the individual components evaporation in E85, with the emphasis on the effects of impinging distance, wall temperature and injection pressure on the mixture formation. The results show that although the larger impinging distance rarely influences the evaporation before impingement, it facilitates the E85 evaporation process after impingement due to the better atomization by low boiling point (LBP) components (e.g. ethanol). The effects of cold-start wall temperature on the E85 impinging spray are greatly dependent on the impinging distance. Under the small impinging distance, the ethanol and low boiling point (LBP) components of gasoline evaporate faster after impingement at higher wall temperature, but such effect becomes absent under long impinging distance. Therefore, the smaller impinging distance shows a higher sensitivity to wall temperature. Lower wall temperature leads to a high wall heat transfer, thus potentially extend the evaporation time at the near wall region along the spray axis. At either impingement timing or initial impingement, the LBP component with high latent heat evaporates fast, leading to a decrease of surrounding gas temperature. Increasing injection pressure helps to greatly improve the evaporation of the high boiling point (HBP) component in gasoline under cold-start impingement. The evaporation ratio of E85 increases up to about 18% ∼ 30% than that of E100 at the initial impinging period under cold-start condition.

Investigation on enhancement of energy conversion of H2/CH4 fueled combustion in a micro tube with multi-channel-outlet

To improve the flame stability and energy utilization of micro-generators, experimental and numerical investigations of a micro-thermophotovoltaic fueled by H2/CH4 are conducted. The coupling effects of multi-channel-outlet setting of micro tube and CH4 addition on combustion characteristics and thermal performance are compared and analyzed. The results reveal that the multi-channel-outlet of the tube significantly enhances the flame stability of premixed H2/CH4/air and the heat transmission via flow recirculation in the combustion chamber. It also notes that 2.5–10 % CH4 addition effectively affects the flame regime and extends the high-temperature zone to obtain a higher radiation temperature. Burners with larger dimensions achieve a lower wall temperatures but attain a higher radiant power, which can be improved with the extension of the multi-channel-outlet length and burner size. And radiant power increases with increasing fuel flow rate, while energy efficiency is reduced. So, the burner with properly multi-channel-outlet setting and a small fraction of CH4 addition is beneficial to improve system thermal performance. It especially obtains that the burner C6-12 has the highest radiant power of 39.6 W at Vf = 640 mL/min and 7.5 % CH4, and burner C6-20 achieves the highest radiant efficiency of 44.1 % at Vf = 410 mL/min.

On paradoxical phenomena during evaporation and condensation between two parallel plates.

Kinetic theory has long predicted that temperature inversion may happen in the vapor-phase for evaporation and condensation between two parallel plates, i.e., the vapor temperature at the condensation interface is higher than that at the evaporation interface. However, past studies have neglected transport in the liquid phases, which usually determine the evaporation and condensation rates. This disconnect has limited the acceptance of the kinetic theory in practical heat transfer models. In this paper, we combine interfacial conditions for mass and heat fluxes with continuum descriptions in the bulk regions of the vapor and the liquid phases to obtain a complete picture for the classical problem of evaporation and condensation between two parallel plates. The criterion for temperature inversion is rederived analytically. We also prove that the temperature jump at each interface is in the same direction as externally applied temperature difference, i.e., liquid surface is at a higher temperature than its adjacent vapor on the evaporating interface and at a lower temperature than its adjacent vapor on the condensing interface. We explain the interfacial temperature jump and temperature inversion using the interfacial cooling and heating processes, and we predict that this process can lead to a vapor phase temperature much lower than the lowest wall temperatures and much higher than the highest wall temperature imposed. When the latent heat of evaporation is small, we found that evaporation can happen at the low temperature side while condensation occurs at the high temperature side, opposing the temperature gradient.

An ultralow-temperature cascade refrigeration unit with natural refrigerant pair R290-R170: Performance evaluation under different ambient and freezing temperatures

The global demand for ultralow-temperature (ULT) refrigeration units gets greatly promoted, for storage, transportation, and distribution of COVID-19 vaccines. In this work, a ULT freezer is developed with a cascade refrigeration system (CRS) utilizing environmentally friendly refrigerants R290 and R170. The performance of the ULT freezer is experimentally evaluated under different ambient temperatures Tamb and freezing temperatures Tfreezing. The result shows that once the refrigeration system starts, the freezer enters a pull-down period and then reaches stable or periodic on-off operation. The monitored temperatures present drastic variations in the low-temperature cycle (LTC) and show a relatively stable start-up process in the high-temperature cycle (HTC). The monitored temperature rises when Tamb increases from 16 °C to 32 °C. The increasing Tamb brings about a larger temperature drop crossing the pre-cooled condenser, cascade heat exchanger, and high-temperature condenser, and a smaller temperature reduction through the anti-condensation. As Tfreezing decreases from −60 °C to −86 °C, the suction/discharge gas temperatures increase in the low-temperature compressor, while the other monitored temperatures reduce. The largest temperature non-uniformity in the freezer is 9.19 °C, and the lowest wall temperature can reach −90.52 °C. With Tamb ranging from 16 °C to 32 °C, the power consumption of the freezer increases from 896 W to 912 W. When Tfreezing varies from −60 °C to −86 °C, the CRS’s consumed power reduces from 804 W to 904 W. The present freezer can easily obtain low temperatures e.g., −81 °C, and reach a lower temperature, such as −86 °C with proper improvements to reduce cold loss.

Effect of T-shaped fin arrangements on the temperature control performance of a phase change material heat sink

The control of operating temperature in phase change material (PCM) heat sinks is restricted by their low thermal conductivity. The present study proposes a novel T-shaped fin strategy, targeting to improve the thermal management capability of PCM heat sinks in the field of electronic devices, i.e., the originality behind is that heat flux from the heated surface can be transported to the solid phase region by T-shaped fins over a longer distance. Based on the background of photovoltaic cell thermal management, a numerical calculation was conducted to investigate the temperature control performance of PCM heat sinks with different T-shaped fin layouts, which was compared with heat sinks without fins and with rectangular fins. The key discoveries indicate that the melting front in the heat sink cavity was severely influenced by the fins, which first suppressed and then enhanced natural convection in the melting process and improved the PCM temperature uniformity. Compared to the unfinned and rectangular fin systems, the employment of T-shaped fins could reduce melting time respectively by up to 25.5% and 14.5% (achieved by case 8), and lower wall temperature respectively by maximum values of 18.9 °C and 4.9 °C (obtained by case 12).

A detailed multidimensional simulation for the cold start process of a gasoline direct injection engine using a fractal engine simulation model

The cold start process for a gasoline direct injection (GDI) engine was studied through multidimensional simulations using a Fractal Engine Simulation (FES) model integrated into CONVERGE CFD. The simulations were focused on the very first firing cycle in the cold start process, as most of the hydrocarbon emissions derive from this cycle. The dramatically changing engine speed and low wall temperatures in the cylinder present challenges for simulation. It turned out that the FES model was able to predict the in-cylinder pressure traces for all four cylinders for the first firing cycle, and gave good agreement with the experimental measurements under these extremely transient conditions. More comprehensive analysis demonstrated the causes for the different behavior of the combustion in different cylinders: more injected fuel and a higher induced tumble ratio in cylinder 3, the first to fire, led to a higher average equivalence ratio and better fuel distribution, which resulted in the highest peak pressure; the worse fuel distribution from the lower tumble ratio, together with the lower normalized turbulence, caused weaker combustion in cylinder 4; the peak pressures were similar and on the low side for cylinders 2 and 1, mainly determined by the low average equivalence ratio. An improved strategy with multiple injections was proposed, with the first two in the intake stroke and two later injections in the compression stroke rather than one injection during each stroke. Although the total injected fuel was the same as the baseline strategy, more fuel evaporated due to the enhanced fuel evaporation from both droplets and wall films, resulting in a higher average equivalence ratio in the cylinder. The peak cylinder pressure and IMEP rose by 33.3% and 8.4%, respectively, using the 4-injection strategy compared to the baseline 2-injection strategy, and the 4-injection strategy was verified by the experiments, as well.