Decrease In Wall Temperature Research Articles

Investigation of heat transfer characteristics and optimization design of platen superheaters in the furnace of a 350 MWe supercritical CFB boiler

With the scale-up of circulating fluidized bed (CFB) boilers, the size of the platen heating surface within the furnace increases. Deformation, crack, and tube explosion of the platen heating surface threaten the safe and stable operation of CFB boiler. Large wall temperature deviation and overtemperature of partial tubes of heating surfaces are two of main reasons resulting in above problems. To identify the causes of large wall temperature deviation and decrease them, the heat transfer characteristics and wall temperature distribution uniformity of platen superheaters in the furnace of a 350 MWe supercritical CFB boiler were studied by experimental tests, and the structure of the high temperature superheater was optimized by numerical simulation to improve the temperature distribution uniformity. The steam parameters and wall temperatures of the platen superheaters were measured at boiler loads of 50 % BMCR, 75 % BMCR, and 100 % BMCR. The heat transfer coefficient, heat flux, and wall temperature uniformity of the platen superheaters were analyzed. At 100 % BMCR boiler load, the average heat transfer coefficient and heat flux of the medium temperature superheater IIs were 165.5 W/(m2·°C) and 56 kW/m2, respectively; while those of the high temperature superheaters were 179.7 W/(m2·°C) and 53.2 kW/m2, respectively. The wall temperature distribution uniformity among six medium temperature superheater IIs or six high temperature superheaters was relative good, while the temperature distribution uniformity of a single platen superheater required improvement. An optimized structure of the high temperature superheater was obtained from eight structures by numerical simulation, achieving a steam temperature deviation of less than 8 °C at the boiler load of 100 % BMCR.

Simulation study on heat and mass transfer characteristics within tubular moving bed heat exchangers

In the field of waste heat recovery, tubular moving bed heat exchangers (MBHEs) hold great promise as equipment capable of effectively handling large-sized granular materials with high temperature. To further clarify the heat and mass transfer characteristics within tubular MBHEs, this study firstly conducted detailed investigations on the contributions of different heat transfer mechanisms. The result shows that heat radiation (∼45 %) and gas film heat conduction (∼42 %) control the heat transfer processes involving tube walls, with an inlet particle temperature of 600 °C, tube wall temperature of 50 °C, and particle descent velocity of 2 mm/s. These contributions can be influenced by the inlet particle temperature and the tube wall temperature, rather than the particle descent velocity. Additionally, for a specific particle, its heat transfer state is highly depended on where it is located. Subsequently, the time-varying and space-varying characteristics of the heat transfer processes involving tube walls are studied. The results suggest that the time-varying and space-varying characteristics are inherent in tubular MBHEs due to the difficulty to achieve complete stability. Meanwhile, temporal fluctuations intensify with an increase in the inlet particle temperature, a decrease in wall temperature, or an increase in particle descent velocity. The result of power spectral density (PSD) indicates that the heat transfer processes exhibit a degree of chaotic behavior due to the nonlinear property of sub-models, as well as the randomness in particle feeding and contacts. The space-varying characteristic is mainly determined by the inlet particle temperature and the tube wall temperature, rather than the particle descent velocity. This characteristic can be observed in both the axial and circumferential directions, with the circumferential variation being more pronounced than the axial variation. Furthermore, a double-tube MBHE, a topic that has received scant attention in previous research, is investigated in comparison to a single-tube MBHE to explore the differences in heat and mass transfer between the two configurations. It indicates that under the same boundary conditions, there are differences in heat transfer results, which are attributed to the interactions between two vertically arranged tubes. These interactions are reflected in the reconstruction of particle flow patterns and the effect altering the temperature distribution for the particles or fluid surrounding the tubes. The obtained results provide valuable insights into the mass and heat transfer behaviors of particles and fluids within tubular MBHEs, laying a foundation for advancing the development of efficient tubular MBHEs.

Thermo-fluidic and entropy generation analysis of MPCMS in straight, segmental, and rib-groove microchannel heat sinks: A numerical investigation

The efficient management of thermal systems is crucial for various technological applications, necessitating innovative cooling strategies to enhance performance and reduce entropy generation. Microchannels have emerged as a promising solution due to their high heat transfer rates and compact size. This study investigates the thermofluidic characteristics and entropy generation in straight, segmental, and rib-groove microchannels utilizing de-ionized water and microencapsulated phase change slurry (MPCMS) as working fluids. Employing numerical analysis, the investigation integrates a mixture model for the base fluid and micro-encapsulated phase change material (MPCM) using an equivalent heat capacity approach to incorporate the phase transition of the MPCM (composed of paraffin with a Ti shell). Operational parameters include an inlet fluid velocity ranging from 0.575 to 3.45 m/s, heat flux varying between 15 and 30 W/cm2, and MPCM volume fractions of 5% to 20%. The ANSYS Fluent Solver, a finite volume method using a multigrid solver, was used to control the simulation by species transport. Notably, segmental channels exhibit a notable decrease in wall temperature of 29.7% and 26.3% compared to straight and rib-groove channels, respectively, under constant heat flux and flow rate. However, the effectiveness of MPCM in enhancing thermal performance was greater in straight and rib-groove channels, surpassing segmental channels by 2.5% and 3.28%, respectively, under constant heat flux. On analyzing the irreversibility, segmental channels exhibit higher frictional entropy but lower thermal entropy generation due to the fin structure, resulting in an overall reduction in total entropy generation. Moreover, segmental microchannels, when utilizing MPCMS, demonstrate superior performance in reducing irreversibility by 60.66% and 49.6% compared to straight and rib-groove channels, respectively. Total entropy generation increases with higher velocity and heat flux, while it diminishes with higher MPCM concentration across all configurations.

Effects of wall temperature on hypersonic shock wave/turbulent boundary layer interactions

Wall temperature has a significant effect on shock wave/turbulent boundary layer interactions (STBLIs) and has become a non-negligible factor in the design process of hypersonic vehicles. In this paper, direct numerical simulations are conducted to investigate the wall temperature effects on STBLIs over a 34° compression ramp at Mach number 6. Three values of the wall-to-recovery-temperature ratio (0.50, 0.75 and 1.0) are considered in the simulations. The results show that the size of the separation bubble declines significantly as the wall temperature decreases. This is because the momentum profile of the boundary layer becomes fuller with wall cooling, which means the near-wall fluid has a greater momentum to suppress flow separation. An equation based on the free-interaction theory is proposed to predict the distributions of the wall pressure upstream of the corner at different wall temperatures. The prediction results are generally consistent with the simulation results (Reynolds number Reτ ranges from 160 to 675). In addition, the low-frequency unsteadiness is studied through the weighted power spectral density of the wall pressure and the correlation between the upstream and downstream. The results indicate that the low-frequency motion of the separation shock is mainly driven by the downstream mechanism and that wall cooling can significantly suppress the low-frequency unsteadiness, including the strength and streamwise range of the low-frequency motions.

Thermodynamic performance and flow structures analysis of supercritical CO2 applied in a regenerative cooling channel mounted with mini-ribs

Supercritical CO2 (SCO2) is regarded as a promising supplementary coolant for traditional regenerative cooling at extremely high flight speed (Ma ≥8) due to its superior heat and mass transfer capability. Using sCO2 as the coolant, mini-ribs is introduced to the rectangular regenerative cooling channel to solve the difficulties in transferring enough heat from near walls to core mainstream at a heat flux of 5 MW/m2 in this paper. With the thermophysical properties near the supercritical point interpolated in the solver, flow structures and heat transfer performance in the cooling channels are obtained using a validated k-ω SST turbulence model. Effects of pitch ratios, channel materials and acceleration and buoyancy effects are also investigated in terms of the local bulk heat transfer coefficients, Fanning friction factors and entropy generation numbers by heat transfer and fluid flow. Overall, the existing of mini-ribs improves the heat transfer coefficient and reduces the entropy number by heat transfer at the cost of the increased fluid friction and the corresponding entropy number. Compared with the smooth channel, the overall enhanced factor can reach 1.73 with an overall thermal performance factor of 1.43 and the maximum wall temperature decreases by 27.57 % in the case of P/e = 10. When the solid channel material has higher thermal conductivity, it leads to a more uniform temperature distribution and lower entropy generation number by heat transfer with the enhanced transverse heat conduction within solid walls which also leads large heat transfer coefficients. Larger accelerations result in stronger heat transfer and larger fluid friction by strengthening the buoyancy effect which is more obvious when the state approach the critical point. This work provides a good reference for the application of supercritical CO2 in a regenerative cooling channel of scramjet engine at extremely high heat flux.

Performance assessment of a geometrical modification on thermosyphon heat pipe with the assistance of novel convergent-divergent truncated cones for energy recovery systems

Heat pipe technology has evolved significantly in addressing thermal management challenges. Still, existing heat pipe designs have limitations in efficiently optimizing heat transfer, particularly in scenarios with spatial constraints and variable heat loads. To overcome these challenges, this study deals with incorporating truncated cones in different forms, such as convergent and divergent, in the thermosyphon heat pipe. The parametric analysis and the performance of this research highlight the ability to handle varying heat loads and determine that a 1:1:1 ratio between the evaporator, adiabatic, and condenser sections of 300 mm in total length thermosyphon heat pipe may be expected to meet spatial constraints efficiently. Placement of the convergent truncated cone in the evaporator and the divergent truncated cone in the condenser helps improve the evaporation and condensation performance by 62.07% and 108%, respectively, when compared to the conventional geometry due to the increasing vapour and liquid velocities. Interestingly, the wall temperature decreases by 11.9%, and the thermal resistance is reduced, increasing the heat transfer coefficient by 85.68% compared to the uniform geometry thermosyphon heat pipe. Hence, these findings provide a path for promising applications in heat recovery and thermal management systems.

Effect of bulk viscosity on the hypersonic compressible turbulent boundary layer

The impact of bulk viscosity is unclear with considering the increased dilatational dissipation and compressibility effects in hypersonic turbulence flows. In this study, we employ direct numerical simulations to conduct comprehensive analysis of the effect of bulk viscosity on hypersonic turbulent boundary layer flow over a flat plate. The results demonstrate that the scaling relations remain valid even when accounting for large bulk viscosity. However, the wall-normal velocity fluctuations $v_{rms}^{\prime \prime }$ decrease significantly in the viscous sublayer due to the enhanced bulk dilatational dissipation. The intensity of travelling-wave-like alternating positive and negative structures of instantaneous pressure fluctuations $p_{rms}^{\prime }$ in the near-wall region decreases distinctly after considering the bulk viscosity, which is attributed mainly to the reduction of compressible pressure fluctuations $p_{c,rms}^{+}$ . Furthermore, the velocity divergence $\partial u_{i} / \partial x_{i}$ undergoes a significant decrease by bulk viscosity. In short, our results indicate that bulk viscosity can weaken the compressibility of the hypersonic turbulent boundary layer and becomes more significant as the Mach number increases and the wall temperature decreases. Notably, when the bulk-to-shear viscosity ratio of the gas reaches a few hundred levels ( $\mu _b/\mu =O(10^2)$ ), and mechanical behaviour of the near-wall region ( $\kern 0.06em y^+\le 30$ ) is of greater interest, the impact of bulk viscosity on the hypersonic cold-wall turbulent boundary layer may not be negligible.

Effects of chloride ion concentration on porous surfaces and boiling heat transfer performance of porous surfaces

Porous surfaces are a common modified surface used to enhance boiling heat transfer. In this paper, a method for preparing gradient porous surfaces is presented, that is, magnetic stirring is added to the electrolyte and the cathode surface to be deposited is placed vertically downward. Within the Cl ion concentration range of 30–140 mg/L, increasing the Cl ion concentration results in lager pore sizes. When the Cl concentration was greater than 120 mg/L, the average pore size of the surface gradually increased from the center to the edge, and the number of pores gradually decreased from the center to the edge. All surfaces showed superhydrophilicity. The critical heat flux of Sample#3 is 158.72 W/cm2, which is 67% higher than the smooth surface. The wall superheat of Sample#3 was only 15 °C in the critical state. This porous surface has plenty of nucleation sites. These nucleation sites are gradually activated when the heat flux increases. At low heat fluxes, heat transfer on these porous surfaces presents a disadvantage because the nucleation sites are not fully activated. At high heat fluxes, these porous surfaces show a decrease in wall temperature. Consequently, the porous surface prepared in this paper has good boiling heat transfer at high heat flux.

Study of the Relationship Between Temperature Change and Energy Transfer in Thermodynamic Processes in Buildings

In this paper, the relationship between room temperature and outside temperature, wall temperature, system operation, and energy consumption is investigated through mathematical modeling and simulation experiments. The thermodynamic parameters of a typical room are used in the study, and the characteristics of the heating and cooling system are considered. 1. This paper analyses the variation of room temperature, wall temperature, switching state, and heating power with time. The results show that the room and wall temperatures are influenced by the external temperature and system operation, while the switching state and heating power are regulated by the room temperature. The correlation between temperature and heating power is quantified by calculating the correlation coefficient matrix. The results show that there is a positive correlation between room temperature and heating power, while there is a negative correlation between wall temperature and room temperature and heating power. 2. This paper investigates the effect of external temperature on room temperature and wall temperature. The results show that an increase in external temperature leads to a decrease in room temperature and wall temperature. In addition, it is found that the indoor temperature is more sensitive to changes in external temperature within the range of changes in external temperature. 3. It is based on the steady-state solution curves between temperature and external temperature, as well as thermodynamic plots of system operation and energy consumption. The visual presentation of the effect of external temperature on the system provides a reference for optimizing building energy use and designing efficient heating and cooling systems. This study provides insight into the relationship between temperature change and energy transfer in building thermodynamic processes, guiding for achieving sustainable energy utilization and reducing environmental impacts. It is of great significance for optimizing the design of building energy systems and improving the efficiency of energy use.

Experimental examination of the effect of thermal barrier coating in a 2-stroke gasoline engine on recovery performance in exhaust waste heat recovery system with thermoelectric generator and engine performance

In internal combustion engines, while the decrease in in-cylinder wall temperature increases the amount of energy passing towards the cooling system, it decreases temperature and pressure at the end of compression and accordingly reduces thermal efficiency of the engine. Moreover; combustion chamber elements are exposed to chemical corrosion and deformation as a result of uncompleted combustion in the cylinder. With thermal barrier coating application, the energy lost in cooling system will decrease and exhaust temperatures will increase in 2-stroke engines. Placement of a waste heat recover system with thermoelectric generator into the exhaust system can provide exhaust waste heat recovery and increase thermal efficiency of the engine. There is no study in the literature, which examines waste heavy recovery systems with thermal barrier and thermoelectric generator altogether. In this article, yttria stabilized zirconia coating has been applied by means of plasma spraying method in the combustion chamber of a 2-stroke gasoline engine, and effect of thermal barrier coating application on engine performance and exhaust waste heat recovery system has been examined experimentally. According the results of the experiment, maximum engine power has increased by 6.23%, maximum engine torque has increased by 6.02%, and specific fuel consumption decreased by 5.56%. While the exhaust temperatures have increased by 78.5 °C at maximum by means of thermal barrier coating application; by means of thermal barrier coating application with waste heat recovery system with thermoelectric generator, a higher waste heat recovery has been obtained by 64.8%.

Pool boiling heat transfer of ammonia outside a single tube with fin structures: Hysteresis phenomena and boiling enhancement

Pool boiling heat transfer of ammonia outside a single tube was investigated at three saturation pressures of 6.15, 8.57, and 11.7 bar (saturation temperature of 10, 20, and 30 °C) at the heat fluxes ranging from 1.2 to 72.9 kW/m2. Hysteresis in a boiling curve and boiling enhancement were examined on both the smooth tube with machined roughness and two enhanced tubes with low fins and re-entrant cavities. All three tubes have outermost diameters from 15.68 to 15.87 mm and the same length of 796 mm. A large hysteresis between increasing the heat flux and decreasing the heat flux was observed on the smooth tube, and the hysteresis increased as the saturation pressure decreased. The boiling hysteresis of ammonia was reasonably explained from the thermodynamic analysis based on Clausius-Clapeyron and Young-Laplace equations. Heat transfer coefficients on the enhanced tubes were increased by 47% to 105% (Low-fin) and 103% to 159% (Turbo-E) compared with the smooth tube at the saturation pressure of 8.57 bar. However, the hysteresis itself was increased on the enhanced tubes owing to the wall temperature decrease along the fin. Hydrophobic coating and the enhanced tube having re-entrant cavities with cavity mouths of a few micrometers were suggested to mitigate the hysteresis phenomena in the pool boiling of ammonia.

Wall cooling effect on spectra and structures of thermodynamic variables in hypersonic turbulent boundary layers

The wall cooling effect on the spectra and structures of thermodynamic variables are investigated in hypersonic turbulent boundary layers. The density and temperature can be divided into the acoustic and entropic modes based on the Kovasznay decomposition. The intensities of the pressure and the acoustic modes of density and temperature attain the maximum values near the wall, while those of the entropy and the entropic modes of density and temperature achieve their primary peaks near the edge of the boundary layer. In the near-wall region, the pressure and the acoustic modes of density and temperature are significantly enhanced when the wall is strongly cooled, which can be attributed to the appearance of the travelling-wave-like alternating positive and negative structures. Moreover, the intensities of the entropy and the entropic modes of density and temperature become stronger near the wall as the wall temperature decreases, due to the appearance of the streaky entropic structures (SES). The SES are mainly caused by the advection effect of the strong positive wall-normal gradient of the mean temperature associated with ejection and sweep events. It is also found that the profiles of the intensities of the entropy, density and temperature are similar to each other far from the wall, which is mainly due to the reason that the entropic modes are dominant in the fluctuating density and temperature in the far-wall region. The acoustic modes of density and temperature only have significant contributions in the near-wall region.

Effect of Cantor fractal fin arrangements on the thermal performance of a photovoltaic-phase change material system: An experimental study

Phase change materials (PCMs) could be utilized to regulate the temperature of photovoltaic (PV) panels, thereby enhancing electrical efficiency. However, the performance of PCMs is limited by their low thermal conductivity. A novel Cantor fin based on fractal theory is put forward in this study to be applied for thermal property enhancement in a PV-PCM system. An experimental investigation regarding the influence of structural characteristics and configurations on the performance of the PV-PCM system was implemented using lauric acid as the temperature control medium. The performance of Cantor fractal fins was compared to that of rectangular fins with an equal volume and a finless cavity. The experimental results indicated that the adoption of fins augmented the uniformity of temperature distribution during the melting process of lauric acid compared to the PV-PCM system without fins. It also enhanced the heat transfer rate from the PV panel to the PCM. Specifically, using Cantor fins resulted in a maximum wall temperature decrease of 9.1 °C. Generally, Cantor fins outperformed rectangular fins in terms of raising even temperature dispersion in the PCM, lowering the temperature of the PV panel, and enhancing wall temperature uniformity. Furthermore, as compared to the vertical configuration, the horizontal arrangement enhanced the PCM temperature uniformity and electrical efficiency despite decreasing the wall temperature uniformity.

Numerical study on the heat transfer characteristics of hydrocarbon fuel in the cooling channel filled with porous media

Regenerative cooling channel is only heated by a single wall in practical applications, resulting in serious thermal stratification. Under the action of porous media, the wall temperature decreases, which avoids the heat transfer deterioration caused by velocity distortion near the high temperature wall. Compared with the cooling channel without porous media, the porous media improves the homogeneity of the coolant heat sink, and the physical and chemical heat sink non-uniformity coefficient at the outlet is close to 0 with the change of aspect ratio. The increase of the height of the cooling channel increases the heat transfer distance in the height direction, which increases the heat transfer resistance of the heat flux and makes the uniformity of the heat sink worse. The positive feedback between the coolant flow rate and the non-uniform heat flux on the parallel filled channel wall can be effectively suppressed. The porous media greatly reduces the radial transfer shortens the axial distance between the upper and lower walls maximum temperature difference, and reduces the thermal stress of channel during the cooling process.

Experimental study on cooling characteristics of an air-cooled evaporative flame holder under combustion conditions

The flame holder needs an integrated design with the cooling structure in advanced ramjet or afterburner combustors. Experimental studies were carried out on the air-cooled evaporative flame holder with impingement-film-slot composite cooling structure under combustion conditions. The effects of blowing ratio M (0.54–2.74) and fuel-air ratio FAR (0.0016–0.0027) on the cooling characteristics of the flame holder were studied under mainstream inlet temperature T = 600 K, inlet Mach Number Ma = 0.15 and on-duty combustion conditions. The results show that the air-cooled evaporative flame holder can stabilize the flame after adding cold air. When the blowing ratio M increases, the wall temperature decreases and the cooling effectiveness increases. The average cooling effectiveness is only 0.237 at M = 0.54 and can reach 0.595 at M = 2.74. The distribution of cooling effectiveness on the inner surface of the flame holder is similar at different blowing ratios. Along the flow direction of the inner surface, the first row of cooling structure A uses the impingement jet and the guide ring to form the initial cooling film. The cooling effectiveness of the first row A is from 0.352 to 0.433 at M = 1.89. The next row B downstream the impingement holes and guide ring uses discrete film holes and has a high cooling effectiveness ranging from 0.578 to 0.815 at M = 1.89. The row C with discrete film holes downstream of B is close to the combustion recirculation zone and has a higher heat load. The cooling effectiveness of row C is 11.7%–23.5% lower than that of row B. The last row D with slot holes is at the trailing edge heated by the hot gas in the recirculation zone. The protective effect of the cooling film is poor in row D and the cooling effectiveness is only about 0.268–0.276 at M = 1.89. In the spanwise direction, the cooling effectiveness is higher near the cold air inlet and lower away from the cold inlet. The increase of the fuel-air ratio (FAR) mainly affects the combustion temperature and wall temperature, and has little effect on the cooling effectiveness. When the fuel-air ratio increases from 0.0016 to 0.0027, the average cooling effectiveness varies from 0.502 to 0.558. As the fuel-air ratio increases, the distribution pattern of cooling effectiveness at all measuring points has little change.

Numerical Investigation of the Heat Transfer Characteristics and Wall Film Formation of Spray Impingement in SCR Systems

This work established a numerical model to investigate the heat transfer characteristics and wall film formation of spray impinging on the wall in the selective catalytic reduction (SCR) system. The model is developed by the Eulerian–Lagrangian approach, where the Lagrangian approach is used to represent the spray generated by a commercial non-air-assisted pressure-driven injector and the Eulerian approach is adopted to represent exhaust gas. The Stochastic Kuhnke Model is applied to spray/wall interaction. The model considers relevant processes, which include mass transfer, momentum transfer, heat transfer, droplet phase change, spray/wall interaction, and wall film formation. The numerical results compared with that of the experiment indicate that the model can accurately estimate the heat transfer characteristics of the wall surface during the spray impingement. Based on the numerical results, the causes of the spray local cooling effect and the rapid cooling effect are analyzed. The correlation between the critical transition temperature and the critical heat flux temperature for wall film formation is derived from the trends of wall temperature and heat flux. In this work, the Stochastic Kuhnke Model is applied and compared with the Kuhnke Model, which proves that it can improve the disadvantage of sudden change during the wall film formation. When the wall temperature is below the critical transition temperature, the wall film mass is sensitive to the wall temperature and increases as the wall temperature decreases.

Two-Dimensional Modelling of Sludge Heat and Mass Transfer in a Paddle Dryer

In this paper, we propose a concise but general two-dimensional model based on the penetration model in order to simulate the heat and mass transfer and drying kinetics of sludge in the paddle dryer. The sludge control equations were developed to simulate the heat and mass transfer of sludge and the penetration model was introduced to describe the mixing of sludge. Compared to the experimental results in the existing literature, the drying kinetics simulated simulated by the present model were in good agreement with the experimental data under various operation conditions. The effects of four key parameters on drying kinetics in the paddle dryer were studied. The results showed that the water content increased with the increase of sludge flowrate and the decrease of wall temperature and sludge density. Furthermore, the stirring velocity had little effect on the drying kinetics of the sludge. The present study contributes to the better understanding of the sludge drying kinetics and provides guidance for the optimal design of industrial paddle dryers.

Numerical investigation of pyrolysis coking characteristics of supercritical hydrocarbon fuels in sinusoidal cooling channels

Carbon deposits in the high-temperature cooling channels of aero engines can lead to severe heat transfer deterioration. So it is significant to reduce the amount of carbon deposit and thus improve the safety of aero-engine operation. In this paper, a two-dimensional axisymmetric CFD model with coupled pyrolysis and coking reactions is developed, and the unsteady formation of carbon is directly simulated by the dynamic mesh technique. The pyrolysis, coking, flow and heat transfer characteristics of supercritical n-decane in smooth and sinusoidal channels during heated for 20 min were studied comparatively. The results show that the coking mass in the sinusoidal channel is basically maintained at 66% of the smooth channel, and the coking layer thickness was relatively declined by 20 μm, which is attributed to the lower precursor concentration and wall temperature in the sinusoidal channel. Specifically, due to a thinner boundary layer and a larger heat transfer area, the maximum difference in fuel conversion rate is 24.4%, with a maximum increase in Nu of 70 and a relative decrease in wall temperature of 85 K. Besides, the flow characteristics are mainly reflected in a significant increase in turbulence at the expense a pressure drop of 15.74 kPa.

Numerical Investigation of the Effect of Surface Wettability and Rotation on Condensation Heat Transfer in a Sludge Dryer Vertical Paddle

In this paper, the applicability of advanced heat transfer enhancement technology to a paddle dryer was discussed. A computational fluid dynamics (CFD) method was used to simulate condensation heat transfer on the inner surface of a dryer paddle. The effect of surface wettability and rotation on condensation heat transfer and droplet behavior was studied. The results showed that the present CFD model could properly simulate the condensation process on a vertical surface. With a decrease in the contact angle, the filmwise condensation turned into a dropwise condensation, which resulted in a significant increase in heat transfer coefficient and provided an approximately 5% increase in evaporation rate for the paddle dryer by changing the wettability of the inner surface of the paddle. Additionally, with a change in rotational angular velocity, heat transfer performance was almost unchanged under the filmwise condensation condition. However, rotational motion might cause a decrease in wall temperature and the equivalent evaporation rate under the dropwise condensation condition. Only a 2.4% increase in the equivalent evaporation rate was found in dropwise condensation with rotation, which indicated that changing the wettability inside the paddle could not be an effective means to enhance the heat transfer and drying efficiency of a rotating paddle dryer.

The Influence of Thrust Chamber Structure Parameters on Regenerative Cooling Effect with Hydrogen Peroxide as Coolant in Liquid Rocket Engines

Liquid rocket engines with hydrogen peroxide and kerosene have the advantages of high density specific impulse, high reliability, and no ignition system. At present, the cooling problem of hydrogen peroxide engines, especially with regenerative cooling, has been little explored. In this study, a realizable k-epsilon turbulence model, discrete phase model, eddy dissipation concept model, and 10-step 10-component reaction mechanism of kerosene with oxygen are used. The increased rib height of the regenerative cooling channel causes the inner wall temperature of the engine increases, the average temperature of the coolant outlet decreases slightly, and the coolant pressure decreases. The overall wall temperature decreases as the rib width of the regenerative cooling channel increases. However, in the nozzle throat area, the wall temperature increases, the average coolant outlet temperature decreases, and the coolant pressure drop increases. A decrease in the inner wall thickness of the regenerative cooling channel results in a significant decrease in the wall temperature and a small increase in the average coolant outlet temperature. These findings contribute to the further development of the engine with hydrogen peroxide and can guide the design of its regenerative cooling process.