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To meet the Chinese government's energy-saving and emission-reduction policies, flexible peak regulation is necessary for traditional coal-fired boilers. Flexible peaking leads to large changes in boiler load, which affects the safety of the boiler water wall. In this paper, a 1000 MW ultra-supercritical unit was tracked for three years, and effective data were selected to study the temperature characteristics of the water wall under flexible peak regulation. The results show that the lower the load, the greater the temperature fluctuation of the water wall. The temperature distribution of the spiral water wall is more uniform. The position of the temperature valley value of the rear spiral water wall was found, and the load of more even temperature distribution was also found. The temperature change of the front vertical water wall was the most complex of all the water walls. The 643.9 MW load case showed different behavior to the temperature distribution of the water wall. The side water walls were heated evenly under the different loads. The characteristics of the temperature distribution of the side vertical water wall were found through statistical analysis. The fitting equation for the change rule of the temperature is presented. The higher the load, the better the equations. Finally, this paper gives some advice on how to avoid temperature deviation in the water wall, and the detailed research highlights the safe running of water walls. [ABSTRACT FROM AUTHOR]

For the first time pilot castings of an aluminum alloy into a patented mold with an evaporative-condensation cooling system are carried out with extraction of the resulting cylindrical billet obtained with a diameter of 62 mm. It is shown that the flow rate of cooling water to a mold may be reduced by a factor of 2–3 by increasing the temperature of the water leaving the condenser to t = 60–80 °C. Adjustments are made to the design calculation of wall temperatures, taking into account preliminary heating of the crystallizer and experimental values of temperatures. The state of the surface of the workpieces obtained is analyzed in two pouring modes. Calculation and comparison of heat supplied to and removed from the mold, and thickness of a billet skin are performed. [ABSTRACT FROM AUTHOR]

In an internal combustion engine, interactions of fuel droplets and heated walls can significantly affect the combustion process and engine performance. The formation and characteristics of secondary droplets from drop-wall interactions are functions of various factors such as fuel properties, impact velocity, ambient conditions, and wall temperature. Understanding the impact behavior is important to optimize the distribution of the fuel-air mixture for efficient and clean combustion and to develop a comprehensive spray-wall interaction model. In this study, three-dimensional smoothed particle hydrodynamics (SPH) simulations are performed to investigate the interactions of fuel droplets with a heated wall at atmospheric and elevated pressures over a range of Weber numbers (We). The SPH model is validated using available experimental data. Secondary atomization is characterized by using size distributions for different fuels. The resulting droplets vary in size, where secondary droplets are mostly below 7 µm in diameter. Following these cases, this paper qualitatively describes the impact process and proposes empirical correlation relating the mean secondary droplet size to ambient pressure in the film-boiling regime. Postimpingement vaporization characteristics are also analyzed and compared for fuels with drastically different vapor pressures. [ABSTRACT FROM AUTHOR]

To evaluate the rationality of the cooling system design for a four cylinder engine, computational fluid dynamics method was used to analyze the engine cooling system from layout design, system flowrate determination, flow field of the cooling system and engine temperature field. The optimization design of adding a throttling notch to the cylinder block water jacket was proposed to solve the problem of significant differences in coolant flow velocity in the cylinder block water jacket. The results show that the coolant flow velocity uniformity in cylinder block water jacket is significantly improved, and the wall temperature of each cylinder liner can be reduced by 1~3 °C. The thermal balance test results at an engine speed of 12 000 r/min show that the maximum difference in cylinder temperature is about 4 °C. The oil temperature is about 129 °C, which is controllable (less than 140 °C), indicating that the cooling system of the four cylinder engine is designed reasonably to ensure uniform cooling of each cylinder and oil cooling. [ABSTRACT FROM AUTHOR]

The rebound and adhesion characteristics of high-temperature alumina droplets upon collision with walls at varied temperatures, are pivotal for the accurate calculation of slag deposition and two-phase flow in solid rocket motors. This study employs an experimental system for high-temperature alumina droplet-wall collisions to gain insights into collision characteristics. The study procures rebound/adhesion regime maps for alumina droplet-wall collisions at different wall temperatures, which suggests that droplets with larger R e n (normal Reynolds number) propensity towards adhesion under the same Ohnesorge number. Further observations include the expansion of the rebound region as the wall temperature incrementally rose from 25 °C to 1000 °C. In view of these, an energy predictive model considering the heat conduction between the droplet and the wall is developed. This model aims to predict the rebound/adhesion of high-temperature alumina droplets colliding with walls at different temperatures. The predictive model's results are then fit to obtain a rapid predictive formula suitable for quick prediction. Both the energy predictive model and the rapid predictive formula undergo verification via 40 validation experiments, consequentially achieving a prediction accuracy of 95.0 %. • Collision result regime maps under walls of 25∼1000 °C are obtained. • A rapid predictive formula suitable for quick prediction is obtained. • Collision result predictive model is established with a prediction accuracy of 95 %. [ABSTRACT FROM AUTHOR]

There is a complex transition phenomenon in the flow field of a typical hypersonic vehicle, which has a significant impact on the performance of the vehicle. The effects of Mach number and wall temperature on the transition of HyTRV were studied by numerical simulation methods. The self-developed software of the research group was used to carry out numerical calculations. The range of Mach number was 3~8, and the range of wall temperature was 150~900 K. Firstly, the hypersonic corrections of the γ-$\mathop R\limits^ \sim $eθt transition model and the SST turbulence model were carried out. The pressure gradient coefficient correction and the high-speed cross-flow correction were introduced into the γ-$\mathop R\limits^ \sim $eθt transition model, and the compressibility corrections of the closure coefficients β* and β of the SST turbulence model were carried out. Then, the grid independence verification was carried out, and the modified numerical method and software platform were confirmed by comparing with experimental results. Finally, the effects of Mach number and wall temperature on the transition law of the HyTRV boundary layer were studied. The results show that the transition area is mainly concentrated on both sides of the upper surface and the center line of the lower surface. With the increase of the incoming Mach number, the starting position of transition on the upper and lower surfaces is greatly backward, and the turbulent zone is greatly reduced, but it still exists. At the same time, the friction coefficient of the laminar flow zone on the upper surface increases continuously, and the friction coefficient of the turbulent zone on the lower surface decreases. As the wall temperature increases, the starting position of transition on the upper and lower surfaces shifts forward, then rapidly shifts backward, and finally the turbulent zone almost disappears.

Copyright of Journal of Shanghai University / Shanghai Daxue Xuebao is the property of Journal of Shanghai University (Natural Sciences) Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The gas temperature measurements at the level of the inner wall of Ranque vortex tubes of the square and circular cross sections with variations in the cold mass fractions for both tubes showed the presence of three characteristic zones of temperature growth, which do not change over a wide range of the cold mass fraction. These zones are typical when analyzing the flow in a vortex tube within the framework of the flow crisis concept. [ABSTRACT FROM AUTHOR]

To ensure the measuring accuracy of the wall temperature, the outer wall temperature measurement values by using three kinds of thermocouple welding methods were analyzed and evaluated in the paper, including single-point flush-mounted in the wall groove method, single-point insert-mounted in the wall groove, and outer surface direct welding method, based on the application of a tube-in-tube condensing heat exchanger. And the impacts of silver, tin, and thermal resistance adhesive as filling materials on wall temperature measurement were also investigated, and the results were compared to that obtained without filling materials. The results showed that the wall temperatures measured by the three welding methods were lower than the theoretically calculated value. And the wall temperature measured by the outer surface direct welding method was lowest under the same experimental conditions. The wall temperatures measured by single-point flush-mounted and insert-mounted in the wall groove methods were also affected by different welding filling materials. It was found that the greater the thermal resistance of filling materials, the smaller the heat loss. By analyzing the reasons for the low measured value of wall temperature, a new wall temperature measurement method was developed to improve the accuracy of the current measurement method. Meanwhile, the outer wall temperature measurement experiments of vertical and horizontal heat transfer tubes were carried out to validate and calibrate the improved outer wall temperature measurement method. The results showed that the average outer wall temperature deviation measured by the improved wall temperature measurement method ranged from −0.82% to +2.29% for vertical tubes and −4.75% to −1.44% for horizontal tubes, and the improved measurement method had good measurement accuracy.

To meet the Chinese government’s energy-saving and emission-reduction policies, flexible peak regulation is necessary for traditional coal-fired boilers. Flexible peaking leads to large changes in boiler load, which affects the safety of the boiler water wall. In this paper, a 1000 MW ultra-supercritical unit was tracked for three years, and effective data were selected to study the temperature characteristics of the water wall under flexible peak regulation. The results show that the lower the load, the greater the temperature fluctuation of the water wall. The temperature distribution of the spiral water wall is more uniform. The position of the temperature valley value of the rear spiral water wall was found, and the load of more even temperature distribution was also found. The temperature change of the front vertical water wall was the most complex of all the water walls. The 643.9 MW load case showed different behavior to the temperature distribution of the water wall. The side water walls were heated evenly under the different loads. The characteristics of the temperature distribution of the side vertical water wall were found through statistical analysis. The fitting equation for the change rule of the temperature is presented. The higher the load, the better the equations. Finally, this paper gives some advice on how to avoid temperature deviation in the water wall, and the detailed research highlights the safe running of water walls.

The transition from laminar to turbulent flow in a hypersonic boundary layer is modeled using an intermittency-based linear combination approach. A simplified transition model like this enables a quick assessment of aero-thermal loads and the overall flight efficiency of high-speed vehicles during the initial design phase by weighting purely laminar and turbulent flow results on the basis of an empirically calculated intermittency. The transition model presented within this work includes an empirical model to account for Mach number, Reynolds number, wall temperature and pressure gradient effects on turbulent spot growth based on available turbulent spot studies in the literature. A validation of the transition model is carried out for a number of different test cases and a methodology to extend the model to generic geometries is presented to enable a more general application. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Ordnance Equipment Engineering is the property of Chongqing University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The operation of an experimental steel mold with a double-circuit cooling system in producing a cylindrical billet from Pb–Sn alloy was studied. The workpiece was extracted after solidification. Before pouring the melt, the mold was heated by an electric heater of 1370 W electric power. The measured temperatures of the mold walls, cooling water, and workpiece were used to calculate the heat transfer in the mold, the crust thickness of the workpiece, and the heat balance. The main similarity criteria for calculating the parameters of steel casting were determined from the parameters of the casting process and the mold. The surface quality of a cylindrical billet was assessed. [ABSTRACT FROM AUTHOR]

Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (Tw), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the Tw, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m2K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the Tw, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m2K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux. [ABSTRACT FROM AUTHOR]

Abstract There is a awakening interest in applying bioconvective nanofluids in permeable media. As such, this article aims to numerically investigate boundary layer flow over a plate embedded in a porous medium filled with nanofluid with motile gyrotactic microorganisms. Consequence, mass and heat transmission efficiency in engineering, technological, and industrial domains is improved. Thus, heat and mass transmission with impacts of radiation and activation energy on the horizontal porous plate are analyzed with microorganisms. Ordinary differential equations are established from nonlinear leading Partial Differential Equations by implementing similarity variables. After that, the resultant ordinary differential equations are numerically tackled employing the BVP4C method, build-in package of MATLAB. From tables, it is evident that when the velocity parameter is one, skin friction is zero, which exhibits no opposition at the fluid-solid surface. The impacts of pertinent parameters upon nondimensional boundary layer outlines are deliberated. It is worth mentioning here that the impact of the magnetic field parameter ( 1 ≤ M ≤ 3 ) on velocity is drawn, and the graph depicts that the velocity is diminished by increasing the value of the magnetic parameter. It is because when the magnetic field is given perpendicular to the liquid’s surface, the generated magnetic impact causes the liquid to flow with a resistive form of force, which reduces its speed for the solutal resistance is increased by activation energy ( 1 ≤ E ≤ 3 ) but decreased by Lewis number ( 1 ≤ L e ≤ 3 ) , and chemical reaction constant ( 1 ≤ σ ≤ 3 ) . Finally, current inquiry can helpfully enhance thermal conductivity, viscosity, convective heat transmission, and thermal diffusivity linked to those base fluids. The current investigation reveals that the Lewis number and chemical reaction constant tend to decay thickness of concentration boundary layer. [ABSTRACT FROM AUTHOR]

Compressible wall-modeled large-eddy simulations of Mach 8 turbulent boundary-layer flows over a flat plate were carried out for the conditions of the hypersonic wind tunnel at Sandia National Laboratories. The simulations provide new insight into the effect of wall cooling on the aero-optical path distortions for hypersonic turbulent boundary-layer flows. Four different wall-to-recovery temperature ratios, 0.3, 0.48, 0.71, and 0.89, are considered. Despite the much lower grid resolution, the mean velocity, temperature, and resolved Reynolds stress profiles from the simulation for a temperature ratio of 0.48 are in good agreement with those from a reference direct numerical simulation. The normalized root-mean-square optical path difference obtained from the present simulations is compared with that from reference direct numerical simulations, Sandia experiments, as well as predictions obtained with a semi-analytical model by Notre Dame University. The present analysis focuses on the effect of wall cooling on the wall-normal density correlations, on key underlying assumptions of the aforementioned model such as the strong Reynolds analogy, and on the elevation angle effect on the optical path difference. Wall cooling is found to increase the velocity fluctuations and decrease the density fluctuations, resulting in an overall reduction of the normalized optical path distortion. Compared to the simulations, the basic strong Reynolds analogy overpredicts the temperature fluctuations for cooled walls. Also different from the strong Reynolds analogy, the velocity and temperature fluctuations are not perfectly anticorrelated. Finally, as the wall temperature is raised, the density correlation length, away from the wall but inside the boundary layer, increases significantly for beam paths tilted in the downstream direction. [ABSTRACT FROM AUTHOR]

In this study, the convective heat transfer of supercritical carbon dioxide in a long vertical mini-pipe has been investigated numerically. The numerical solution has been performed with the finite volume method and by developing a CFD code. The pipe has a length of 5.5 m and a diameter of 1 mm which is exposed to a constant heat flux at the wall with values of 300, 400, 500, and 600 W/m2 or step changes. In addition to the wall heat flux, the effects of gravity and flow direction have also been examined. Furthermore, some differences between the results of laminar and turbulent flows have been addressed. The results show that in the laminar flow, unlike the turbulent flow in the improvement regime of heat transfer, the system's thermal performance increases with increasing the wall heat flux, while in the deterioration mode, the two have similar behavior. Moreover, in part of the downward flow, reverse flow occurs, and its length can be understood by using the negative amount of wall shear stress. Furthermore, the thermal efficiency of the supercritical carbon dioxide is better at the upward flow and near the critical point than the constant property flow. In addition, from the applied stepped wall heat flux, it is concluded that the deterioration can be partially controlled or reduced by correctly determining the location of the step or any wall heat flux variations.

This paper presents a novel physics-infused reduced-order modeling (PIROM) methodology for efficient and accurate modeling of nonlinear dynamical systems. The PIROM consists of a physics-based analytical component that represents the known physical processes and a data-driven dynamical component that represents the unknown physical processes. The PIROM is applied to the aerothermal load modeling for hypersonic aerothermoelastic (ATE) analysis and is found to accelerate the ATE simulations by two to three orders of magnitude while maintaining an accuracy comparable to high-fidelity solutions based on computational fluid dynamics. Moreover, the PIROM-based solver is benchmarked against the conventional proper-orthogonal decomposition/kriging surrogate model and is found to significantly outperform the accuracy, generalizability, and sampling efficiency of the latter in a wide range of operating conditions and in the presence of complex structural boundary conditions. Finally, the PIROM-based ATE solver is demonstrated by a parametric study on the effects of boundary conditions and rib supports on the ATE response of a compliant and heat-conducting panel structure. The results not only reveal the dramatic snapthrough behavior with respect to spring constraints of boundary conditions but also demonstrate the potential of the PIROM to facilitate the rapid and accurate design and optimization of multidisciplinary systems such as hypersonic structures. [ABSTRACT FROM AUTHOR]

Hypersonic boundary-layer stability has significant importance in vehicle design and successful operation. This paper investigates the stabilization effects of local wall cooling on the hypersonic boundary layers over a 5 deg half-angle blunt cone with a nose radius of 0.0254 mm. We employed a high-order-accurate flow solver to calculate the steady flow for a freestream Mach number of 6.0 and a unit Reynolds number of 25.59×106/m. In simulations, we considered partial wall cooling, entire wall cooling, and adiabatic wall scenarios. Furthermore, we examined partial cooling parameters such as strip location, length, and temperature profiles. We calculated the growth rates, phase speed, and N-factor diagrams using a linear stability analysis. The results showed that complete wall cooling destabilizes the boundary layer. However, the cooling strip upstream of the synchronization point stabilized the boundary layer by damping the disturbances. The longer cooling strip further stabilized the boundary layer. The cooling strip placed downstream of the synchronization point destabilized the boundary layer. [ABSTRACT FROM AUTHOR]

The effect of wall temperature on the ignition of vapor-phase spray impingement has been studied extensively, but the mechanism of the liquid-phase case is not clear. Thus, the visualization experiment was conducted at wall temperatures of 363–673 K using Mie-scattering, shadowgraph, and direct photography methods in a constant volume combustion vessel. The results reveal that, compared with the vapor-phase wetting-wall, when the liquid fuel impinges on the cold wall, the threshold temperature of the autoignition increases greatly to 363 K. Furthermore, the wall temperature strongly affects the evaporation and fuel-air mixing under the liquid impingement case. As the wall temperature increases from 423 K to 573 K, the enhanced evaporation results in a significant reduction in the spray radius, height, and area. However, the evaporation is inhibited and the spray parameters increase when the wall temperature further rises to 623–673 K. This characteristic may be caused by the Leidenfrost effect, which is the main difference from the vapor-phase impingement. Too low wall temperature (below 520 K) easily leads to misfire and unstable ignition, and the threshold temperature of the autoignition is greatly increased, while a higher wall temperature exceeding the Leidenfrost temperature is also not conducive to rapid and stable ignition. With the increase of wall temperature, the ignition delay first shortens from 4.1 ms to 3.2 ms gradually and then prolongs to 3.6 ms again, while the flame area and intensity first increase and then decrease, and the transition temperature is 573 K. When liquid wetting-wall inevitably appears in a heavy-duty diesel engine operating at low temperatures, it is necessary to raise the wall temperature moderately to ensure stable ignition. However, it should not exceed the Leidenfrost temperature, otherwise, evaporation will slow down and ignition will worsen due to the Leidenfrost effect. [ABSTRACT FROM AUTHOR]

Micro-diffusion flames have received much research attention due to their use in micro-energy systems. Experiments are performed with liquid heptane fuel at room temperature to assess the effects of fuel flow rates on micro-diffusion flame characteristics, wall temperature, and evaporation phenomena. At low fuel flow rate (≤10 µl/min), a steady, laminar flame with a spherical shape is formed, the maximum wall temperature recorded was 1050 K, a stable flow regime was observed with a dynamic meniscus, and the liquid fuel evaporated completely at the interface. At a medium fuel flow rate (30 µl/min), an unsteady flame with periodic patterns was formed, the maximum wall temperature dropped to 970 K, and the flow regime became unstable with liquid droplets in vapor regime. At a high fuel flow rate (≥50 µl/min), an explosive flame with an erratic pattern was obtained, the maximum wall temperature substantially decreases to 890 K, and an explosive boiling regime prevails with nucleate bubbles in the downstream region. The flame instability relies on meniscus oscillation and liquid droplets variation at low and high fuel flow rate, respectively. [ABSTRACT FROM AUTHOR]

An experimental cylindrical mold of patented design with electric heating and a two-circuit cooling system, including an evaporation–condensation circuit, for producing cylindrical billets from high-alloy steels was manufactured and tested. A mandatory element of this design is its preheating to a temperature of 120–150°C before pouring steel and supplying cooling water to the system. The temperatures of the cooling water and walls are recorded automatically. A design calculation of mold cooling at a minimum heat flux of 0.5 MW/m2 is performed. [ABSTRACT FROM AUTHOR]

The problem of redesigning the wide walls of the CCM-2,3 molds at the oxygen-converter plant of the Magnitogorsk Iron and Steel Works is solved due to the need to change over from copper plates with drilled channels to copper or bronze cassette-type plates with slotted cooling channels and hardening coating. The redesign of the mold walls should not affect the loop circuit for supplying cooling water and pouring metal into a twin mold. When creating the thermal model of the new wide wall with slotted cooling channels, the existing thermal design methods were analyzed. The temperature drops of the cooling water and the dependence of the average surface temperature of the cooling channels on the pouring rate and the speed of water in the channels are obtained. The temperatures of the wall without coating, with nickel–cobalt and with steel coating 3 mm thick depending on the metal pouring rate for different speeds of cooling water in the channels are determined. A thermal model of the wide plate of the mold of new design is created. It allows analyzing the thermal parameters of the wall with slotted channels and hardening coating for different operating conditions of the CCM. It is established that the temperature of the surface with coatings is 1.3 to 1.5 times higher than without coating. In addition, the coating can significantly increase the overall thermal resistance of the wall. It is recommended to apply a wear-resistant coating only to the lower, most wearing part of the mold plates. [ABSTRACT FROM AUTHOR]

This work demonstrates high-speed 2D wall temperature measurements occurring during flame-wall interaction (FWI) within a narrow channel. Such measurements are essential to understand transient wall heat transfer and flame behavior occurring within micro-combustors and designated engine crevices. Wall temperature is measured using the phosphor Bismuth-doped Scandium vanadate (ScVO 4 :Bi3+). ScVO 4 :Bi3+ exhibits a short phosphorescence lifetime (∼ 2 µs at room temperature), enabling kHz measurement rates. ScVO 4 :Bi3+ also exhibits a high temperature sensitivity, which yields single-shot precision < 0.5 K within the temperature range of 295 – 335 K. A frequency-doubled Ti:Sapphire laser emitting light at 400 nm is used to excite ScVO 4 :Bi3+, and wall temperature is measured within a ∼ 22 × 22 mm2 region with 380 µm spatial resolution. Phosphor thermometry and CH* imaging are combined at 1 kHz to measure the spatiotemporal dynamics of the flame and wall temperature (T wall) within a 2 mm crevice passage in a fixed volume chamber designed for heat transfer studies. Measurements describe T wall signatures associated with transient FWI events, including unique T wall features associated with wrinkled flame fronts. For our operating conditions, T wall associated with flame cusps consistently exhibit temperatures 5 – 20 K lower than flame crest regions. The most extreme difference is seen for large cusp formation, where local wall cooling is noticeable at the trough of the flame cusp. This cooling feature may be caused by intrinsic flame/flow instabilities, which locally and temporally cool the wall. [ABSTRACT FROM AUTHOR]

The structure of the trapped-vortex cavity and radial flameholder can maintain stable combustion under severe conditions, such as sub-atmospheric pressure and high inlet velocity. This article reports a complete study of combustion characteristics for this design. The flow field of the physical model was obtained by numerical simulation. The pilot combustion characteristics, including the combustion process, combustion efficiency, and wall temperature distribution, were studied by experiments. The pilot combustion can be divided into three modes under different fuel flow rates and inlet conditions. In "cavity maintained (CM)" mode, pilot flame exists at both sides of the cavity zone, rotating with the main vortex. In "cavity-flameholder maintained (CFM)" mode, the combustion process occurs both inside the cavity and behind the flameholder. While in "flameholder maintained (FM)" mode, the cavity will quench, and the combustion is maintained by the radial flameholder only. Due to the difference in the flow field, the flame pattern and propagation direction vary under different combustion modes. The combustion efficiency, influenced by combustion modes, shows an increase-decrease-increase curve. The wall temperature distribution is also affected; the cavity wall temperature decreases under large fuel flux while the temperature of the burner-back plate continues to rise to a maximum value. [ABSTRACT FROM AUTHOR]

Copyright of Nuclear Safety is the property of Nuclear & Radiation Safety Center and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Behaviors of H 2 -enriched CH 4 MILD combustion at low wall temperatures are studied. • H 2 co-firing can stabilize MILD combustion at lower wall temperatures (e.g., 690 K). • H 2 addition causes departure from MILD combustion at high wall temperatures. • Lowering wall temperature changes the major NO route of H 2 from thermal to NNH/N 2 O. • A higher thermal efficiency is achieved in MILD combustion using H 2 instead of CH 4. Hydrogen-enriched methane moderate or intense low-oxygen dilution (MILD) combustion is a promising technology that offers an attractive pathway to achieve low NO x and CO 2 emissions in furnaces and boilers. This paper numerically investigates the influence of hydrogen-blending ratios ( X H 2 = 0–100 %) and wall temperature (T wall = 1500–300 K) on establishment, NO formation/reduction mechanisms, and heat transfer behaviors of hydrogen-enriched methane MILD combustion with non-preheated air in a laboratory-scale furnace. Results show that hydrogen addition increases the peak Damköhler number from 7.5 to 20.3, which could cause departure from MILD combustion at high wall temperatures. However, lowering T wall can slow down the MILD reaction rate and thus reduce Damköhler number closer to unity, which is beneficial for achieving hydrogen-enriched methane MILD combustion. Interestingly, as X H 2 is increased from 0 to 100 %, the threshold wall temperature for achieving MILD combustion is extended from 1170 to 690 K. In other words, MILD combustion could be sustained/stabilized under low T wall conditions (encountered in boilers) when burning pure hydrogen or fuels with a high percentage of hydrogen. Moreover, as T wall is lowered, the major NO formation pathway is changed from thermal-NO to NNH and N 2 O-intermediate in pure hydrogen MILD combustion; also, H reburning via the pathway of NO → +H HNO → +H NH → +NO N 2 becomes more important in reducing NO at a lower T wall level. As X H 2 is increased from 0 to 100 %, the radiative heat transfer is enhanced by 29.4 % while the convective heat transfer is reduced by 16.8 %, ultimately resulting in an increase of 15.0 % in the total heat transfer (T wall = 1300 K). Along the furnace sidewall, the distribution of heat flux has a low value near the burner exit, peaks in the downstream furnace, and declines further downstream close to the furnace exit in MILD combustion with non-preheated air. [ABSTRACT FROM AUTHOR]

Four building arrays, each comprising 25 building models, were utilized to investigate the effects of cool materials on building surface temperatures, indoor temperatures, and street microclimates in Guangzhou, China. The experiments are divided into three groups. The first group of the experimental models is used to investigate the cooling effect of the roof using cool materials on the roof surface and indoor environment. The results show that using cool materials on roofs can significantly reduce roof surface and indoor temperatures of the top floor by 13.5 °C and 3.9 °C at noon under an average global solar radiation (GSR) of 727 W/m2. The second group of experiments included four arrays: one control group without cool materials and three other arrays applying cool materials on the third, second, and first floors, respectively. Using cool materials on the façades of the third, second, and first floors reduced the indoor temperature by an average of 1.9 °C, 0.8 °C, and 0.5 °C (average GSR = 404 W/m2), respectively. As a result, higher floors receive more solar radiation, and cool materials show better cooling capacity. The third group is used to investigate the effect on street air temperatures when cool materials are used on all building façades. Overall, the effect of cool materials on the street air temperature is very small, with a maximum reduction of only 0.2 °C in the street air temperature. [Display omitted] • Four building arrays consisting of 100 building models were used. • The cooling effect of cool materials on indoor, building surface, and street air were analyzed. • Using cool materials on high-floor façades has a better cooling effect than on low-floor ones. • Cool materials on the top floor reduce indoor temperatures by 1.4 °C more than the first floor. • The effect of cool materials on the street air temperature is minimal, less than 0.2 °C. [ABSTRACT FROM AUTHOR]

Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (Tw), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the Tw, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m2K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the Tw, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m2K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux.

We carry out a parametric study of hypersonic impinging shock-wave/turbulent-boundary-layer interactions (SBLIs) at hypersonic conditions (Mach number 6.0) by means of numerical simulation of the Reynolds-averaged Navier-Stokes (RANS) equations, with the eventual goal of establishing wall temperature and Reynolds number effects. Comparison with available direct numerical simulation and experimental data shows that RANS is capable of predicting the main features of SBLI, namely, typical size and distribution of the wall properties. A large number of flow cases were computed to examine the scaling of the global flow parameters over a wide range of wall temperatures and Reynolds numbers. As expected, the interaction zone is reduced as the wall is cooled, and as the Reynolds number is increased. A modified form of the scaling originally introduced by Souverein et al. [Journal of Fluid Mechanics, Vol. 714, 2013, pp. 505-535] is here considered to account for nonadiabatic wall state, which is found to successfully collapse the data to the distributions obtained for adiabatic flow, with some remaining deviations associated with low-Reynolds-number effects. [ABSTRACT FROM AUTHOR]

Aspects of regenerative heat exchange in a piston compressor are investigated. The most thermodynamically advantageous situation occurs when compression approaches an isothermal process. The heat balance equation, which compares the amount of heat released per piston compressor cycle and that removed through the outer surface of the working chamber, was used to develop a method for calculating heat absorbed by working chamber walls in a compressor depending on the duration of compressor operation. The developed procedure provided a basis for parametric analysis to determine the influence of the main operating parameters (pressure ratio in a stage, angular velocity of crankshaft rotation) on the final temperature and heating time of the walls. This procedure can be used for preliminarily estimating the operating time of a piston machine employing regenerative cooling in compressor mode. [ABSTRACT FROM AUTHOR]

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In military and aerospace fields, petrol and aviation kerosene are the preferred fuels for micro power systems due to their reduced carrying load and convenient fuel source. Therefore, n-heptane, an alternative to petrol and aviation paraffin, was chosen as the fuel. A micro quartz glass tube was used as the combustor, and the premixed combustion characteristics of n-heptane/air were investigated using numerical simulations. The method used to improve the internal flow channel includes a built-in cross baffle and scaled backward-facing step. The resulting optimized combustion characteristics, such as internal flow line, wall temperature, CO emission, and wall radiation, have been analyzed. The results indicate that, with a fixed equivalence ratio of 1.0 and a n-heptane mass flow rate of 2.70 mg/s, the built-in cross baffle concentrates and stabilizes the high temperature zone of the flame, resulting in a significant increase in wall temperature. Conversely, the use of the micro tube with scaled backward-facing step creates a reflux zone due to the sudden expansion of the inlet cross-section, which promotes better fuel mixing and flame anchoring. Furthermore, after comparing the output efficiencies of micro tube with different internal structures, it was discovered that enhancing the internal channel of the micro tube is crucial for increasing the wall radiation efficiency., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors. Published by Elsevier Ltd.)

Real gas effects in high entry temperatures are taken into consideration combining flight conditions during hypersonic flight into Earth's atmosphere. Convective stagnation heating loads are estimated from the original full Fay-Riddell formula and compared with other engineering analytical methods. A new engineering correlation formula is proposed for wall stagnation temperature distribution during reentry motion. The efficiency of the applied analysis gives satisfactory results compared with published data of verified experiments and computational codes for the lifting Space Shuttle STS-40 and ballistic Apollo4 capsule missions. [ABSTRACT FROM AUTHOR]

This paper presents a correlation to obtain the surface species concentration on a stagnation point with mass injection for hypersonic flow. The correlation can be used to determine surface recombination heat fluxes and oxidation levels for ablatives or actively cooled heat shields in thermochemical equilibrium flow. It explicitly expresses the wall species concentration as a function of freestream conditions, coolant properties, and the stagnation-point geometry, revealing the direct effect of the aforementioned parameters on the desired quantity. The method was compared against the numerically obtained self-similar solution and showed an accuracy of ±4%. The concentration depends on the boundary-layer edge pressure, temperature, and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas. [ABSTRACT FROM AUTHOR]

An idealized shock/boundary-layer interaction problem that is two dimensional in the mean but with a three-component mean velocity field is studied using large-eddy simulations. The problem isolates some aspects of three-dimensionality while avoiding others and can thus offer insights into the differences between two- and three-dimensional problems. The addition of a crossflow increases the size of the separation bubble quite substantially when normalized by the boundary-layer thickness or the momentum thickness but less substantially when normalized by the displacement thickness. All cases studied are found to have a very thin precursor separation bubble before the point of true separation that extends for at least one boundary-layer thickness but remains fully within the viscous sublayer. Simulations at different Reynolds numbers show that the flow reattaches after the precursor bubble at sufficiently high Reynolds number. [ABSTRACT FROM AUTHOR]

• For L/D ratio of 13, the flow type transforms from a closed one to a transitional one. • The critical wall temperature is approximately 775 K. • As the incoming static temperature drops, the critical wall temperature declines. • Transition caused by the competition between the shear layer and recirculation zone. • Heat transfer is affected by the thickness of boundary layer or recirculation zone. The wall temperature of the high-speed aircraft increases quickly under hypersonic/supersonic incoming flow, which will cause a significant change in the flow structure. To study the transition of the flow type in the supersonic cavity controlled by the wall temperature, numerical simulations are conducted. The cavity length-to-depth ratio (L/D) is varied from 10 to 15, and the wall temperature ranges from 300 K to 1300 K. The results indicate that the type of cavity flow with an L/D ratio of 13 transforms from a closed cavity flow to a transitional cavity flow, when the temperature reaches approximately 775 K. And the transitional temperature rises with the elevated total temperature of the incoming flow. Furthermore, the mechanism of the cavity flow change with wall temperature could be the competition between the recirculation zone and the shear layer in the cavity. The rising pressure with higher wall temperature in the recirculation zone weakens the downward development of the cavity shear layer, preventing it from hitting the cavity floor. As a result, the mass exchange of cavity lip surface, pressure distribution, total pressure recovery coefficient, and heat transfer distribution in the supersonic cavity change dramatically. The critical wall temperature also affected by the sidewall effects and the inflow Mach number. [ABSTRACT FROM AUTHOR]

The helical cruciform fuel (HCF) is designed with the functional and structural integration concept. The function of the conventional spacer grid is replaced by the spiral structures while they are the components of the HCF element. The HCF is an optional design for the new generation of nuclear reactors. In the present work, the subcooled boiling in the 4 × 4 HCF assembly is investigated based on the Eulerian multiphase model and auxiliary models for interphase exchanges. The averaged and localized distributions for the transverse flow, vapor fraction and wall temperature are analyzed, which is of great importance for the reactor safety design. The vapor phase is mainly generated from the elbow, and it flows towards the leeside of the blade. With low quality, the nucleation boiling merely occurs near the elbow and the heat transfer of the fuel assembly is mainly affected by the forced convection. The temperature difference between the blade parts and the elbow parts is over 23.0 °C with the quality of −0.278. The temperature difference between the elbow and the rest part of HCF element gradually decreases by increasing the quality of coolant. The azimuthal wall temperature is almost the same with the quality larger than −0.156, and the temperature difference is less than 0.5 °C. Meanwhile, the effect of twisted angle on the heat transfer of HCF assembly is negligible with larger quality. The Papell correlation provides good prediction accuracy for the current simulation results. A new correlation that has higher prediction accuracy is established, which is applicable effective when P = 15.5 MPa, G = 1.30 × 103∼5.86 × 103 kg/(m2·s), q = 2.29 × 102∼1.37 × 103 kW/m2, x = −0.278∼0. The new correlation may be applied in the thermal-hydraulic analysis of the HCF assembly. [ABSTRACT FROM AUTHOR]

Addressing the cooling challenges posed by multi-heat sources with high heat flux in densely integrated electronic devices remains a pressing concern. A study was conducted to explore the performance of two distinct microchannel heat sinks, with featuring grooves or not, in a series connection. This investigative series connection mode exhibited notable enhancements in performance. Bubble behavior flow patterns within microchannels were observed utilizing a high-speed camera, followed by an analysis of the associated heat transfer mechanisms. The study encompassed the acquisition and detailed analysis of three key parameters, wall temperature, heat transfer coefficient (HTC), and pressure drop, of both heat sinks across diverse series connection configurations. Flow patterns were varied and including bubble flow, elongated bubble formations, and the presence of thin liquid films with steam channels in different positional heat sinks. Remarkably, the series connection exerted a substantial influence on postposition heat sinks compared to their preposition counterparts. Among the investigated series connection modes, with featuring grooves ones, exhibited the most favorable impact on heat sink performance. Notably, the postposition heat sink in this configuration demonstrated a maximum HTC increase of 12.77 kW/(m2K). The distinctive heat transfer mechanisms led to higher HTC and lower wall temperature in postposition heat sinks compared to their preposition counterparts. Findings in this study offer crucial insights into addressing cooling challenges associated with multi-heat sources featuring high heat flux, serving as valuable groundwork for potential solutions in this critical domain. [ABSTRACT FROM AUTHOR]