Outer Wall Temperature Distribution Research Articles

A conjugate heat transfer investigation of outer-wall cooling performance in double-wall vane pressure side

Double wall cooling has significant potential for high-pressure turbine vane cooling technology. Geometric parameters of the double-wall vane play a crucial role in determining the cooling performance. This paper presents a double-wall cooling model with five outer wall thicknesses and three injection angles, using the curvature of the actual pressure side of the engine as a reference. The overall cooling effectiveness is investigated over the range of blowing ratio from 0.5 to 2.5. Results indicate that, at injection angles of 20° and 30°, an outer-wall thickness of 1.5D f maximizes the combined benefits of film cooling and thermal conductivity in solid domains. At an injection angle of 40°, the blowing ratio becomes a determining factor in selecting the optimal outer wall thickness. Additionally, a reasonable arrangement of pin fins promotes a more uniform outer wall temperature distribution. A thinner outer wall at higher blowing ratios provides optimum overall cooling effectiveness.

Supercritical carbon dioxide heat transfer in horizontal tube based on the Froude number analysis

The heat transfer and pressure drop (ΔP) of supercritical fluid are crucial for the safety of advanced power cycles. Here, experiments were performed for sCO2 in a horizontal tube with inner diameter (din) of 10 mm, covering in the ranges of (G): 496.7–1346.2 kg/m2, heat flux (qw): 97.4～400.3 kW/m2 and pressure (P): 7.53–23.51 MPa. Inspired by the similarity of the outer wall temperature (Tow) distributions between the sCO2 flow and those of stratified-wavy-flow in subcritical pressures, the multiphase flow theory is introduced in subcritical pressures to deal with the sCO2 heat transfer. In the two-phase-like (TPL) regime, heat transfer coefficients (HTC) and friction factors (f) of sCO2 are found to significantly deviate from the correlations based on the single-phase flow theory. Further, the vapor-like Froude number FrVL is proposed to develop new correlations for both the heat transfer and the flow resistance covering the whole data range. The present paper establishes a connection between the flow resistance and heat transfer in supercritical pressures, which is important for the design and operation of sCO2 power cycles.

Thermographic Investigation on Fluid Oscillations and Transverse Interactions in a Fully Metallic Flat-Plate Pulsating Heat Pipe

The present investigation deals with the quantification of fluid oscillation frequencies in a metallic pulsating heat pipe tested at varying heat loads and orientations. The aim is to design a robust technique for the study of the inner fluid dynamics without adopting typical experimental solutions, such as direct fluid visualizations through transparent inserts. The studied device is made of copper, and it is partially filled with a water–ethanol mixture (20 wt.% of ethanol). Heat fluxes locally exchanged between the working fluid and the device walls are first assessed through the inverse heat conduction problem resolution approach by processing outer wall temperature distributions acquired by thermography. The estimated local heat transfer quantities are therefore processed to quantify the fluid oscillatory behavior in every device branch during the intermittent flow and full activation regimes, thus providing a deeper insight into the heat transfer modes. After dealing with a further validation of the inverse approach in terms of oscillation frequency restoration capability, the wall-to-fluid heat fluxes referred to each channel are processed by means of the wavelet method. Scalograms and power spectra of the considered signals are presented for a time-based analysis of the working fluid oscillations, as well as for the identification of dominant oscillation frequencies. Fluid motion is then quantified in terms of the continuity of fluid oscillations and activity of channels by applying a scalogram denoising technique named K-means clustering method. Moreover, a statistical reduction of the channel-wise dominant oscillation frequencies is performed to provide useful references for the interpretation of the overall oscillatory behavior. The link between oscillations and transverse interactions is finally investigated. The vertical bottom-heated mode exhibits stronger fluid oscillations with respect to the horizontal mode, with fluid oscillation frequencies ranging from 0.78 up to 1 Hz. Nonetheless, the fluid motion is more stable in terms of oscillation frequency between channels when the device operates in the horizontal orientation probably due to negligible buoyancy effects. Moreover, thermal interactions between adjacent channels are found to be stronger when the oscillatory behavior presents similar features from channel to channel in horizontal orientation. The proposed method for fluid oscillation analyses in fully metallic flat-plate pulsating heat pipes can be effectively adopted to other flat-plate layouts without any need for transparent windows, thus reducing the overall complexity of experimental set-ups and providing, at the same time, a good insight into the inner fluid dynamics.

Investigation of thermal performance and energy conversion in a novel planar micro-combustor with four-corner entrances for thermo-photovoltaic power generators

A novel planar micro-combustor with four-corner entrances is proposed by considering high and uniform outer wall temperature distribution used in micro thermo-photovoltaic generator. For this, a three-dimensional numerical model with detailed chemical mechanism, transport, and heat transfer and a thermo-photovoltaic energy conversion model are developed to examine its performance. Results show that the proposed micro-combustor has excellent wall temperature uniformity because a uniform distribution of high-temperature zones of four individual flames is formed in the combustion chamber. Thermal performance and energy efficiency of the proposed micro-combustor are better than conventional micro-combustor at a low flow rate ( m ) and large equivalence ratio ( φ ). Additionally, the implemented plate-insertion strategy effectively overcome the deterioration of combustor performance under higher m or lower φ . Proposed micro-combustor with plates insertion has the largest wall temperature and energy efficiency, no matter what φ and m are adopted. Plates extend the fluid ravel path and produce some recirculation zones, which promotes heat transfer, preferential diffusion, and flame anchoring, while its pressure loss is larger. Of all the tested cases, the system efficiency reaches a maximum of 6.39%, corresponding to 12.1 W electricity output. This work provides effective combustor design and research insights to develop high-efficiency micro-power system. • A novel micro-combustor with four entrances is proposed for micro-TPV system. • Excellent performance of wall temperature uniformity is obtained. • Inserting plates boosts thermal performance and energy conversion efficiency. • A maximum system efficiency of 6.39% is achieved with 12.1 W electric output.

Influence of hole size and number on pressure drop and energy output of the micro-cylindrical combustor inserting with an internal spiral fin with holes

Targeting at optimizing the energy output and thermal performance of the micro combustor in the application of micro-thermophotovoltaic (MTPV) systems, but the introduction of spiral fins brings higher pressure loss. Thus, a novel design of the micro combustor with the spiral fin opening is developed. The influence of inlet velocities, the hole size and hole number of the spiral fin on the pressure drop and thermal characteristic and energy characteristic are numerically investigated. It's illustrated that the spiral fin opening is conductive to decrease the pressure loss and optimize the outer wall temperature distribution, but has a negative influence on increasing the mean outer wall temperature of the micro combustor and energy output in the MTPV systems. With the increase of the hole size and hole number of the spiral fin, the pressure loss decreases and the outer wall temperature uniformity increases significantly, while the mean outer wall temperature drops and total energy output decreases. The better performance obtains when the micro combustor with spiral fin with four 0.5 mm holes.

Boundary Conditions for Simulations of Fluid Flow and Temperature Field during Ammonothermal Crystal Growth—A Machine-Learning Assisted Study of Autoclave Wall Temperature Distribution

Thermal boundary conditions for numerical simulations of ammonothermal GaN crystal growth are investigated. A global heat transfer model that includes the furnace and its surroundings is presented, in which fluid flow and thermal field are treated as conjugate in order to fully account for convective heat transfer. The effects of laminar and turbulent flow are analyzed, as well as those of typically simultaneously present solids inside the autoclave (nutrient, baffle, and multiple seeds). This model uses heater powers as a boundary condition. Machine learning is applied to efficiently determine the power boundary conditions needed to obtain set temperatures at specified locations. Typical thermal losses are analyzed regarding their effects on the temperature distribution inside the autoclave and within the autoclave walls. This is of relevance because autoclave wall temperatures are a convenient choice for setting boundary conditions for simulations of reduced domain size. Based on the determined outer wall temperature distribution, a simplified model containing only the autoclave is also presented. The results are compared to those observed using heater-long fixed temperatures as boundary condition. Significant deviations are found especially in the upper zone of the autoclave due to the important role of heat losses through the autoclave head.

Effect analysis on energy conversion enhancement and NOx emission reduction of ammonia/hydrogen fuelled wavy micro-combustor for micro-thermophotovoltaic application

Ammonia (NH3) is regarded as an alternative fuel not only as a carbon-free fuel but also as a renewable hydrogen-carrier. It is possible that the safety in micro-combustor can be improved through partial NH3 substitution for hydrogen. However, knowledge of the thermal performance and nitrogen oxides (NOx) emission of ammonia/hydrogen combustion, especially in the micro-combustor, has been insufficient. In order to enhance thermal performance, reduce NOx emission and improve flame stabilization of ammonia/hydrogen fuelled micro-combustors for thermophotovoltaic (TPV) application, three types of micro-combustors with a wavy profile are designed and evaluated. For this, a three-dimensional (3D) numerical model with a detailed chemical reaction mechanism has been verified and applied to assess the thermal performance of the modified micro combustors in terms of the outer wall temperature distributions. The average temperature of these wavy combustors is found to be much higher than that of the conventional smooth combustor, regardless of the hydrogen/ammonia mixture flow velocity. Moreover, the wavy is a more effective measure to improve temperature uniformity when the mixture velocity is greater than 12 m/s. Comparing the flame stability behaviours of hydrogen/ammonia/air blended combustion in both the conventional and the proposed wavy combustors reveals that the blowout limit is effectively broadened. Finally, the effects of 1) hydrogen/ammonia blended ratio and 2) fuel-air equivalence ratio on NOxemissions are examined in detail. It is found that approximately 21.2% of NOx emission reduction could be achieved in the ARC wavy micro-combustor. NOx emission reduction can be gradually improved, as the nitrogen fuel mass ratio is increased. This present research sheds lights on an effective design of a micro-combustor with enhanced thermal performance and reduced NOx emission.

Transient solution of temperature field of conjugate laminar forced convection heat transfer in functionally graded hollow cylinder

AbstractIn this study, the numerical analysis of conjugate heat transfer of laminar flow in a functionally graded hollow cylinder (FGHC) made of metal/ceramic for a two‐dimensional fluid and wall conduction subject to Newton boundary condition is considered. The fluid and FGHC energy equations are coupled through the continuity of temperature and heat flux at the inner wall‐fluid interface while the outer surface is subject to convective heat transfer. The continuity, momentum, and energy equations of the fluid are discretized using the finite volume approach. The effects of fluid and functionally graded material parameters, such as volume fraction index, volume composition, time history, wall‐to‐fluid thermal diffusivity ratio, wall‐to‐fluid thermal conductivity ratio, Biot number, Peclet number, and Prandtl number are investigated on the temperature field in the FGHC. The result shows that on account of the inhomogeneity of the material property, the volume fraction index has a significant effect on the other parameters and the temperature variation along the thickness. The lower the volume fraction index, the higher the inner wall (metal side) temperature, and the temperature gradient along the thickness. However, except for the variation in the wall‐to‐fluid thermal conductivity ratio, the lower the volumetric fraction, the lower the outer wall (ceramic side) temperature distribution.

Investigation on premixed H2/C3H8/air combustion in porous medium combustor for the micro thermophotovoltaic application

The study of premixed H2/C3H8/air combustion in the combustor inserted with porous medium (PM), which involved ten kinds of PM and varied with porosity, wire diameter and pores per inch (PPI), is experimentally and numerically conducted. Moreover, in order to boost the electrical power output and efficiency of the micro thermophotovoltaic (TPV) system, the effects of flow rate and propane blended ratio of the fuel on combustion and the thermal performance are investigated. The results indicate that the flame stability and heat transfer of premixed H2/C3H8/air combustion can be significantly enhanced in PM. The 10% propane blended fuel effectively changes the flame location and temperature, as well as the thermal performance of the combustor and the outer wall temperature distribution, while the thermal properties of the combustor are less decreased with 15% propane added. It is also found that the combustor inserted with PM, which has an areal density of 0.0524–0.0551 g/cm2 (PM #30-0.2, #60-0.14 and #80-0.12) and a porosity of 0.9, achieves the highest mean wall temperature and provides a high and uniform wall temperature at different inlet flow rates. The combustor inserted with PM #30-0.2 achieves the electrical power output 2.02 W and efficiency 1.6% for the micro TPV system under the condition of fuel flow rate 400 mL/min, propane blended ratio 12% and equivalence ratio 0.8.

Effect of Channel Diameter on the Combustion and Thermal Behavior of a Hydrogen/Air Premixed Flame in a Swirl Micro-Combustor

Improving the flame stability and thermal behavior of the micro-combustor (MC) are major challenges in microscale combustion. In this paper, the micro combustions of an H2/air premixed flame in a swirl MC with various channel diameters (Din = 2, 3, 4 mm) were analyzed based on an established three-dimensional numerical model. The effects of hydrogen mass flow rate, thermal conductivity of walls, and the preferential transport of species were investigated. The results indicated that the flame type was characterized by the presence of two recirculation zones. The flame was anchored by the recirculation zones, and the anchoring location of the flame root was the starting position of the recirculation zones. The recirculation zones had a larger distribution of local equivalence ratio, especially in the proximity of the flame root, indicating the formation of a radical pool. The combustion efficiency increased with an increasing Din due to the longer residence time of the reactants. Furthermore, the MC with Din = 2 mm obtained the highest outer wall temperature distribution. However, the MC with Din = 4 mm had a better uniformity of outer wall temperature and large emitter efficiency due to the larger radiation surface. An increase in thermal conductivity boosts the thermal performance of combustion efficiency, emitter efficiency, and wall temperature uniformity. But there is a critical point of thermal conductivity that can increase the thermal performance. The above results can offer us significant guidance for designing MC with high thermal performance.

Heat transfer in a partially filled rotary evaporator

Heat transfer with evaporation in a partially-filled, rotating pipe with water flowing through it is experimentally investigated. The test section is a 32.8 mm ID, 1000 mm long stainless steel pipe with a wall thickness of 1.1 mm. The outer wall temperature distribution is captured using a thermal camera. Uniform wall heat flux (3348–21200 W/m2), water volume flow rate (100–400 ml/min), rotation rate (10–300 RPM) and pipe inclination angle (0°–6°) are varied to study their influence on the heat transfer characteristics of a rotary evaporator. Location of the onset of nucleate boiling is used to study the effect of various parameters on the rotary evaporator operation. With the increase in the pipe inclination angle and wall heat flux, the onset of nucleate boiling is achieved closer to the inlet of the rotary evaporator. Increasing liquid flow rate delays the onset of nucleate boiling for given wall heat flux and inclination angle. Rotation rate has no significant effect on the rotary evaporator performance in the subcooled boiling region. Local heat transfer coefficient along the length of the partially filled rotating test section is reported.

A modified two-dimensional numerical method for prediction of outer wall temperature distribution of rectangular micro-combustors

Two-dimensional (2D) numerical models are frequently adopted to investigate combustion and thermal performances in rectangular micro-channels for micro-thermophotovoltaic and thermoelectric devices. However, large error may exist by applying a simple 2D model. In the present work, the outer wall temperature distributions predicted by 3D model and simple 2D model were compared. The results showed that the maximum relative error of the simple 2D model depends significantly on the aspect ratio (α) of the micro-channel. To be specific, the maximum relative error was >30% for α = 1 and > 10% for 2≤α ≤ 4. However, it was <5% for α ≥ 9. A new 2D model was proposed to modify the underestimated heat loss ratio. The new computational results demonstrated that the maximum relative error of α = 1 decreased to 8.07% and for micro-channels with α ≥ 2, all the maximum relative errors are <5%. In summary, the modified 2D numerical model can achieve a satisfactory prediction with low computation cost.

Experimental investigation on non-premixed CH4/air combustion in a novel miniature Swiss-roll combustor

In the past decades, combustion in miniature Swiss-roll combustors has widely investigated for applications to micro power generation systems. However, all the existing Swiss-roll combustors are designed for premixed combustion mode, in which the flame is prone to flash back or become extinguished at low flow rates. In the present work, a miniature Swiss-roll combustor was designed specifically for non-premixed combustion, which has two inlets for counterflows of methane and air, and two outlets for exhaust gas. Preliminary experimental investigation was conducted on a model combustor with a transparent quartz cover, which demonstrated that this new configuration brings about several merits. First, the flame structure and outer wall temperature distribution under most conditions are almost symmetric. Second, this combustor can run at a very low equivalence ratio (<0.3) and a very small fuel flow rate (˜0.029 L/min). As a result, the wall temperature level is relatively low, which makes this combustor a potential heat source for miniature thermoelectric devices using low-temperature materials such as Bi2Te3. Furthermore, the exhaust gas can be controlled at a very low temperature level (˜390–450 K), which means this micro combustor has a good heat recirculation performance.

Combustion characteristics and thermal performance of premixed hydrogen-air in a two-rearward-step micro tube

Combustion characteristics, flame location and thermal performance of premixed hydrogen-air combustion in two combustors with different outer diameters (OD = 4 mm and OD = 5 mm) are investigated and compared. In order to enhance the flame stabilization and improve the efficiency, the micro combustors are presented with two rearward-steps and various inlet shapes. Verified with experimental tests, a three-dimensional numerical model has been employed to assess the flame location, OH radical and gas temperature distributions. It is observed that the flame location and thermal performance are significantly affected by the structure of the micro combustor. The results indicate that a properly stepped/flared inlet section contributes to the ignition and enhances the preheating of the unburned reactants. The heat recirculation zone behind the rearward-step provides a suitable location for the flame anchoring. In addition, a high and uniform outer wall temperature distribution is beneficial for the efficiency improvement and system application. The combustor with a small size obtains a higher wall temperature and is more sensitive to the change of combustion chamber. However, the combustors with OD = 5 mm gains the better flame stability and working performance with a higher power output, radiation efficiency and system efficiency at a higher mass flow rate. Moreover, the power output and system efficiency of the combustor with two rearward-steps are 2 times higher than that with one step.

Impact of inclination on single phase heat transfer in a partially filled rotating pipe

Heat transfer in a partially-filled, rotating inclined pipe with water flowing through it is experimentally investigated. The test section is a 1000 mm long stainless steel pipe with 32.8 mm inner diameter and 1.1 mm wall thickness. The outer wall is painted black to improve its emissivity. The outer wall temperature distribution is captured using a thermal camera. Uniform wall heat flux (1405–10784 W/m2), water volumetric flow rate (100–830 ml/min), rotation rate (10–300 RPM) and pipe inclination angle (3° and 6°) are varied to study their influence on the heat transfer coefficient. Local heat transfer coefficient along the length of the partially filled rotating test section is reported. While heat transfer coefficient increases with the increase in wall heat flux, liquid volume flow rate and rotation rate, it decreases with increase in inclination angle. A generalised correlation to predict the average Nusselt number is developed in terms of four dimensionless numbers, viz., the flow Reynolds number to capture the effect of the axial fluid flow, rotation Reynolds number to account for the effect of pipe rotation, flow Froude number to take care of the effect of pipe inclination and dimensionless heat flux to incorporate the effect of wall heat flux on the heat transfer coefficient.

Combustion characteristics of premixed hydrogen/air flames in a geometrically modified micro combustor

Main challenges for a fuel efficient micro scale combustion are rooted from size restriction of micro combustors which results with inappropriate residence time of fuel/air mixture and intensified heat losses due to relatively high surface to volume ratio of such devices. One way of increasing energy output of micro combustors is to optimize its geometry by considering simplicity and easy manufacturability. In this study, effect of combustor geometric properties on combustion behavior of premixed hydrogen/air mixtures was numerically investigated. For this purpose, an experimentally tested micro combustor’s geometric properties were modified by establishing a backward facing step which is varying distance from combustor inlet and has varying step height, and adding opposing cavities which are varying distance from combustor inlet and have constant length to depth ratio into the flow area. Modeling and simulation studies were performed using ANSYS Design Modeler and Fluent programs, respectively. Combustion behavior was analyzed by means of centerline and outer wall temperature distributions, amount of heat transferred through combustor wall, conversion ratio of input chemical energy to utilizable heat, and species distributions. Turbulence model used in this study is Renormalization Group (RNG) k-ε. Multistep combustion reaction scheme with 9 species and 19 steps was simulated using Eddy Dissipation Concept model (EDC). Results showed that backward facing step in the flame region alters reaction zone distribution, flame length and shape, and consequently temperature value and distribution throughout the combustor. Lastly cavity was found to slightly increase peak temperature value.

Investigation on the effects of wall thickness and porous media on the thermal performance of a non-premixed hydrogen fueled cylindrical micro combustor

An investigation on non-premixed H2/air combustion in a cylindrical micro combustor has been carried out. The effects of porous media and outer wall thickness on the combustion characteristics, flame location, thermal performance and energy conversion efficiency of the thermo photovoltaic (TPV) system were investigated. For the application of micro TPV system, a high and uniform outer wall temperature distribution is indispensable for the sustaining output, and the high energy efficiency is desirable. The results indicate that the setting of porous media or the increase of outer wall thickness can enhance the heat transfer in micro combustor and affects the flame stability, and the micro combustor with porous media and outer wall thickness b = 0.2 mm obtains the lowest flame location. They are also conducive to the improvement of outer wall temperature and energy efficiency, the outer wall temperature of the micro combustor with a thicker outer wall and porous media is relatively uniform and the energy conversion efficiency of micro TPV system is also improved, the micro combustor with b = 0.6 mm and porous media is more suitable for the application of micro TPV system. Additionally, the external thermal environment also can improve the outer wall temperature profile and the working performance of the micro combustor.

Thermal performance of a meso-scale combustor with electrospray technique using liquid ethanol as fuel

A new meso-scale combustor designed to couple with energy conversion modules is fabricated. The volume of the combustor is of the order of a few cubic centimeters. Liquid ethanol is electrosprayed at a flow rate of 3.50ml/h. The stable flame is observed to be in a disc shape near the mesh of the combustor when the equivalence ratios varying from 0.9 to 1.7 without external heating and catalyst. The temperatures of flame, combustor outer wall and exhausted gas are measured. Flame temperatures are within the range of 1134–1287K. Exhausted gas components are detected by a gas chromatograph and the combustion efficiencies of ethanol are calculated, which are from 51.2% to 92.4%. Heat loss from the combustor wall is evaluated and it accounts for 29.2–43.6% of the input energy. The outer wall temperature distributions along the flow direction are obtained and a heat recirculation zone is found at about 10mm away in the upstream of the mesh, which is beneficial to stable combustion. The mesh in the present combustor is used not only asa collector for gathering charged droplets but also a flame holder. The thermal efficiencies of the combustor are found to vary from 22.0% to 48.8%.

Effect of micro combustor geometry on combustion and emission behavior of premixed hydrogen/air flames

In this study, effect of micro combustor geometry on combustion and emission behavior of premixed hydrogen/air mixtures is numerically investigated. An experimentally tested micro combustor geometry is varied by establishing a cavity or a backward facing step or micro channels inside the combustor. Considering effect of combustor geometry on the amount of heat transferred through wall based on outer wall and combustor centerline temperature distributions, combustion behavior is analyzed. Emission behavior is examined by means of mixing conditions, combustion efficiency and maximum temperature value which are highly bound to geometric properties of a micro combustor. Turbulence model used in this study is Renormalization Group k-ε. For turbulence chemistry interaction, Eddy Dissipation Concept model is used. Multistep combustion reaction scheme includes 9 species and 19 steps. Numerical results obtained from this study are validated with published experimental data. Results of this study revealed that combustion in such combustors can be improved by means of quality of mixing process, residence time, combustor centerline and outer wall temperature distributions, conversion rate of input chemical energy to utilizable heat and emanated NOx levels from combustor outlet with proposed geometric variations.

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NOx profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.