Wall Temperature Distribution Research Articles

Heat transfer correlation of smooth inverted annular film boiling in a circular tube

Experiments on steady film boiling heat transfer are conducted in a circular tube with test conditions covering the range of 10–50 K inlet subcooling, 0.2–2.5 MPa pressure, and 100–1000 kg/m2s mass flux. Through analysis of experimental data and comparison with corresponding predictions by the existing correlations, they are demonstrated to be insufficient for accurately capturing the varying trends of the experimental data within inverted annular flow boiling (IAFB) regime. It is discovered that IAFB is divided into three regions, namely, smooth IAFB, rippled IAFB and turbulent IAFB, by the distribution of wall temperature and its rate of change along axial direction, which is consistent with relevant observations from previous study. Only the vapor film in the smooth IAFB can be attributed to laminar vapor film flow, which leads to the discrepancy in the overall trend of existing correlations and experimental data. As fluid flows downstream, interface interaction due to velocity difference between vapor film and liquid core accounts for the fluctuations. The assumption that transition between smooth and rippled IAFB is result from Kelvin-Helmholtz (K-H) instability accounts for the flow characteristics of vapor film in smooth IAFB. Heat transfer at interface is obtained based on vapor–liquid velocity. Additionally, a new heat transfer correlation for smooth IAFB is proposed. The MAPE of the new correlation is 16.4%, and the RMSPE is 18.5%. The new correlation could more accurately describe the heat transfer characteristic in smooth IAFB.

Numerical simulation and experimental investigation of bubble behaviour during pool boiling in the coiled wire

Under the computational fluid dynamics (CFD) framework, an interface tracking method has been used to perform several multiple simulations of coiled wire boiling in a pool. This study includes simulations of the boiling regime, including the Critical Heat Flux (CHF), ranging from nucleate boiling to film boiling. The purpose of this research was to investigate using coiled wire to increase the rate of heat transfer under constant heat flux conditions. The range of the applied heat flux is 130–1075 kW/m2. This research delves into the formation of bubble shapes, bubble diameter, and bubble departure frequency. The vapour bubble grows in size as the heat flux increases; with a heat flux of q'' = 600 kW/m2, it reaches its maximum measured diameter of about 9 mm. Additionally, the heat transfer coefficients and wall temperature distribution were determined using the simulation findings. The heat flux was gradually increased in the critical heat flux investigation until burnout of the coiled wire was observed and a bubble layer formed. This led to the conclusion that q'' = 1075 kW/m2 represents the CHF for this particular geometry. It was observed that the CHF conditions occur in the coil portion, which emphasises the importance of considering the critical heat flux more carefully while designing this geometry.

Flow Fluctuation during Flow Boiling of Binary Mixtures in High Aspect Ratio Microchannel

Flow boiling performance is affected by several factors, such as channel characteristics and working fluid types. It is found that there is still limited study that discusses the use of binary mixtures combined with high aspect ratio microchannels. The aim of this study is to investigate the flow fluctuation during flow boiling of binary mixtures in rectangular microchannels. Here, a 6 mm width and 0.3 mm depth rectangular channel was utilised, and it represents a hydraulic diameter of 571 μm and an aspect ratio of 20. In the present works, a mass flux of 10 kg m-2 s-1 was used, and the heat flux ranged from 15.2 and 21.0 kW m-2. The image processing technique was applied to track the bubble tail movement. In addition, the thermal camera was utilised to gather the wall temperature distribution of the channel. The preliminary results show that the use of binary mixtures influences the vapour fraction in the channel and the flow fluctuation characteristics. Some differences are observed in terms of wall temperature characteristics. However, the rapid increase of wall temperature is found in the outlet region for high flux cases under all liquid types which suggests the dominance of dry out event.

Experimental Study and Numerical Simulation of Gas Dryer Structure Improvement

The dryer is an important part of the paper drying process, and the uniformity of the dryer wall temperature distribution has an important influence on paper production quality and efficiency. In this paper, improving the temperature uniformity of the traditional gas dryer wall is taken as the research goal, and the distribution trend and uniformity of the traditional gas dryer wall temperature are studied and analyzed, and the structural improvement plan is put forward. On the basis of this, in order to further improve the uniformity of the wall temperature of the improved gas dryer, the optimization scheme of applying endothermic coating in the low-temperature area of the inner wall of the dryer is proposed. The numerical simulation and experimental research methods are used to compare and analyze the temperature uniformity of the wall of the improved gas dryer. The results show that the axial uniformity of the wall temperature of the modified gas dryer is significantly improved. Compared with the traditional gas dryer, the temperature difference of the cylinder wall is reduced from 40 °C to 13 °C, the maximum axial temperature difference of the cylinder wall is reduced by 57%, and the temperature uniformity is increased from 66.7% to 89.6%. Compared with the improved gas dryer, after the endothermic coating is applied to the low-temperature area of the inner wall of the dryer, the temperature difference of the cylinder wall is reduced from 13 °C to 7 °C, the maximum axial temperature difference of the cylinder wall is further reduced by 46%, and the temperature uniformity is increased from 89.6% to 94.4%.

Numerical study of irreversibility analysis on pulsatile flow of blood through a ω-shaped stenotic artery

The mathematical modeling of blood flow in a stenotic blood vessel is very important for understanding the effects of various geometric and rheological parameters on the normal flow of blood. The present study is concerned with the heat and mass transfer in the blood flow of a non-Newtonian fluid through a ω-shaped stenotic arterial segment. The non-Newtonian behavior of blood is described mathematically by employing the constitutive equation of the Herschel-Bulkley fluid. The wall of the artery is considered to be rigid having ω-shaped constriction in its lumen. The mild stenosis assumption is used to simplify the fully coupled momentum, energy, and concentration equations as well as the constitutive equation of the Herschel-Bulkley fluid. Numerical solutions for fluid velocity, temperature, mass concentration, and entropy generation are evaluated by using the explicit finite difference scheme. The effects of various emerging parameters on the velocity, wall shear stress, flow rate, temperature, concentration, and entropy distributions have been analyzed in detail. A comparison between different fluid models has also been presented. It is found that the temperature in the Herschel-Bulkley fluid is marginally lower than that of the Newtonian, Power law, and Bingham fluids whereas an opposite trend is noticed in the case of mass concentration.

Experimental Study of Deformation Risk and Interpolation Analysis of Temperature for Water Walls under Flexible Low-Load Conditions

With the development of renewable energy in power generation, a large number of thermal power plants operate under flexible working conditions. To study water wall deformation risk under flexible low-load conditions, a lab-scale opposed firing boiler was built to measure and analyze the temperature and thermal stress of the water wall under low-load flexible operating conditions with several nontypical burner arrangements. The results showed that flame radiation and flue gas convection heat transfer induced a high-temperature zone. Thermal stress was mainly caused by thermal expansion of the metal and expansion or extrusion deformation of other areas. Also, local thermal stress fluctuations and excessive values appeared under variable conditions. In addition, the burners’ symmetrical arrangement in the center effectively reduced thermal stress fluctuations and overall thermal stress under flexible working conditions. Finally, based on the limited temperature measurement points in this experiment, the optimal power parameter α for the inverse distance weighted (IDW) method and the shape parameter ε for the radial basis function (RBF) method were found to make a reasonable interpolation prediction of the temperature distribution of the water wall. This study highlights the recommended burner arrangements under flexible low-load conditions and the optimal parameters of the IDW and the RBF methods for estimating temperature in guiding safe operation of the water wall.

Investigation of heat transfer at the outer combustor wall on self-sustained thermoacoustic oscillations in a Rijke tube

Thermoacoustic oscillations have plagued combustion systems for decades and are becoming increasingly relevant as engines become more powerful, more compact, and more eco-friendly. These oscillations arise from constructive coupling between unsteady heat release and acoustic pressure waves, and result in strong pressure oscillations that can inhibit flame stability and induce component failure. In this work, a simple numerical model is used to study the effects of the temperature distribution of the outer combustor wall on the thermoacoustic stability of a Rijke tube. A Galerkin series expansion of the linearized acoustic governing equations is coupled with a classical G-equation premixed flame model to determine the minimum energy required to maintain stability for various system configurations, and results are compared against computational fluid dynamics simulations for select cases.

Application of heat transfer at the combustor wall to attenuate self-excited thermoacoustic oscillations

Self-sustained thermoacoustic oscillations (also known as combustion instability) arise in combustors when pressure and heat release fluctuations couple to produce high-energy sound waves. The need for diverse strategies to control these oscillations will increase as future design moves towards highly efficient, lean combustors. In this work, the impact of heat transfer at the combustor wall on the generation of thermoacoustic oscillations is examined through computational fluid dynamics. Characteristics of the thermoacoustic oscillations generated across cases with a range of thermal boundary conditions are compared to an adiabatic reference case. The amplitude of limit cycle oscillations in a Rijke tube is seen to decrease as wall temperatures increase. A stable combustor with a negative growth rate is observed when the full outer wall is heated to 900 K, and a sound pressure level reduction of 75 dB is achieved. The most efficient stable configuration is achieved when only the inlet duct wall is heated to 1000 K, requiring a minimum of 430 W of input power. The present work introduces an alternative stable combustor design and demonstrates that optimizing the temperature distribution of the combustor wall can effectively dampen thermoacoustic oscillations.

Effect of the Injection Structure on Gas Velocity Distribution in a 3D Vertical Oven

Gas injection structures were designed for a vertical oven to improve the gas–solid flow countercurrent structure. This work measured the wall temperature distribution of the vertical oven to reflect gas velocity distribution, and simulated the basic gas–solid flow field. The effects of the number of gas orifice layers and the injection angle on the gas velocity distribution were examined. The results showed that number of gas injection layers had a significant effect on the gas velocity distribution in the lower zone. Compared with the distributions with one or three injection layers, two injection layers produce more uniform gas flow. A small particle size of 6–15 mm increased the bed resistance and solid fraction standard deviation. A nozzle angle of 45° was conducive to increase the gas velocity in the upper zone and forming a more uniform gas distribution.

Enhancement of heat transfer in heat sink under the effect of a magnetic field and an impingement jet

Improving the performance of heat sinks is very important in the development of cooling systems. In this study, the use of a novel combination method [magnetic field impingement jet (MF-IJ)] to improve the convective heat transfer coefficient in a designed heat sink is numerically investigated. To model heat transfer, a steady three-dimensional computational fluid dynamics (CFD) approach is employed. Numerical results including velocity and temperature contours, as well as the distribution of wall temperature of the heat sink and also the convective heat transfer coefficient are analyzed. The results show that the use of ferrofluid (Fe3O4/water) flow with an external magnetic field alone increases the heat transfer coefficient by 10%, while the use of an air impingement jet with pure water and without a magnetic field increases it by 22.4%. By using the MF-IJ method, a 32% enhancement of heat transfer coefficient is achieved compared to the case of pure water flow and without MF-IJ. Based on results, at a Reynolds number of 600, by applying the magnetic field intensities of 400, 800, and 1600 G, the average heat transfer coefficient increases by 5.35, 11.77, and 16.11%, respectively. It is also found that the cooling of the heat sink and temperature distribution is improved by increasing the Reynolds number and the inlet mass flow rate of the impingement jet. For instance, at z = 0.02 m, the application of an impingement jet with mass flow rates of 0.001, 0.004, and 0.005 kg/s results in a respective decrease of 0.36, 1.62, and 1.82% in wall temperature. The results of the current study suggest that the combination method of MF-IJ can be utilized for heat sinks with high heat flux generation as a flow control device.

Spectral Heat Transfer Coefficient for convection

The present work is aimed at developing a methodology to predict convective heat transfer of flow bounded by a non-isothermal wall, which has been a long-standing issue of interest for Newton's law of cooling. A framework is proposed to centre around the Spectral Heat Transfer Coefficients (SHTC). They are effectively global-to-local temperature-heat flux influence coefficients for a small number of low order spectral modes of wall temperature disturbances for a range of physically relevant and numerically resolvable length scales. The framework is implemented with a unified Fourier spectral method applicable to both spatially periodic and non-periodic wall temperature distributions. The Fourier spectral formulation also leads to the zeroth harmonic base mode corresponding to exactly the standard form of Newton's law of cooling, thus the method is directly applicable to both non-isothermal and isothermal cases, as intended. The method formulations in both 2D and 3D settings and the working procedure are presented. All computational case studies for 2D and 3D configurations clearly and consistently demonstrate the validity and effectiveness of the proposed framework methodology and method implementation.

Obstacle Detection in Infrared Navigation for Blind People and Mobile Robots.

The paper is a continuation of the authors' work intended for infrared navigation for blind people and mobile robots. This concerns the detection of obstacles in the person's or mobile robot's trajectory, in particular, the detection of corners. The temperature distribution of a building's internal wall near a corner has been investigated. Due to geometry, more heat will be transferred by conduction so that inside the building, the temperature on the wall will be decreasing towards a corner. The problem will be investigated theoretically and numerically, and the results are confirmed by experimental measurements. The purpose of this research is to help blind people by equipping them with a small infrared camera that warns them when they are approaching a corner inside a building. The same aim is addressed to mobile robots.

Modelling of bubble dynamics on vertical rough wall with conjugate heat transfer

The wall temperature distribution and wettability greatly influence the bubble dynamics and heat transfer process. In this work, lattice Boltzmann (LB) method is implemented to investigate the bubble dynamics on the vertical rough heating wall, where the conjugate heat transfer process is taken into consideration. The interaction between bubble dynamics and boiling heat transfer characteristics is revealed. The results reveal that the bubble motion and the gas–liquid interface are dependent on the wall thermal conductivity. A high wall thermal conductivity will cause the gas film to cover the wall surface and hinder the heat transfer process. Moreover, the influence of the wall mixed wettability on the conjugate heat transfer behaviors is also analyzed. The findings demonstrate that the step wall wettability can promote the nucleation efficiency so as to achieve the enhancement of the wall heat transfer.

Saturated nucleate flow boiling over a horizontal single and multi-cylinder under cross-flow condition

Present work focuses on simulating flow boiling of saturated liquid to reveal associated interfacial dynamics over horizontal heated circular cylinder under vertical flow condition. Varying liquid velocities and heat fluxes are considered for simulation to cover the study of the buoyancy and inertia-dominated mixed regimes. Cross-flow situation around isolated and vertically arranged in-line cylinders are modelled using Volume-of-Fluid (VOF) based interface capturing technique and Tanasawa’s phase change model. In case of a single cylinder, at lower cross-flow rate, individual bubble life cycle can be identified; whereas, at higher cross-flow rate, a continuous but wavy column is formed behind the cylinder due to formation of the stagnation zone and associated interfacial instability. The wall temperature and void fractions distributions from bottom to top of the cylinder differentiate the azimuthally varying heat transfer mechanisms. The inter-cylinder interfacial interaction in case of multi-cylinder is shown with sequential bubble movement illustrations. As moving from the bottom cylinder to top, the bubble size is observed to be increasing.

Numerical analysis of high temperature potassium heat pipe under marine condition

A heat pipe (HP) is a two-phase heat transmission device with very high effective thermal conductivity, extensively employed in multiple industrial fields. In the nuclear engineering area, numerous experimental and numerical studies are widely produced for its thermal behavior performance in high temperatures. In this paper, a HP that potassium as the working fluid with a capillary wick structure was taken into consideration under marine conditions through numerical simulations. The physical properties of potassium and the additional force method of rolling motion are utilized with user-defined functions (UDFs). This CFD model verified with available experimental results, is employed to investigate the influence of HP surface wall temperature distribution and thermal resistance by several parameters including inclination angle, filling ratio (FR), and rolling period. Besides, the charts of phase change and wall temperature distributions are visible with the help of CFD simulation under static and rolling condition. Therefore, the high temperature HP application of design and evaluation is provided as a reference in present work under marine condition.

Experimental results for a MW-scale fluidized particle-in-tube solar receiver in its first test campaign

To advance the development of particle-driven technologies, this study presents experimental results of a megawatt-scale solar receiver installed atop the Themis Solar Tower (France). The whole experimental loop and the solar flux distribution at the receiver aperture are described with the associated instrumentation. The solar receiver is composed of forty 4-meter long metallic tubes, and the irradiated part size is 3 m high and 2.6 m wide. The facility was operated at partial load with a maximum solar power of approximately 800 kW with a particle mass flow rate ranging from 2148 kg/h to 12456 kg/h. The tube wall temperature distribution is presented along with a particle temperature increase that reached 400 °C. The dynamic behavior of the solar loop is detailed, in particular the starting phase. The thermal efficiency of the solar receiver ranges from 35 to 63%. An assessment of the lessons learned during this experimental campaign is offered.

Effect of local wall temperature on hypersonic boundary layer stability and transition

Wall temperature significantly affects stability and receptivity of the boundary layer. Changing the wall temperature locally may therefore be an effective laminar flow control technique. However, the situation is complicated when the wall temperature distribution is nonuniform, and researchers have experimentally found that local wall cooling may delay the onset of transition. We attempt to clarify the physical mechanisms whereby the local wall temperature affects the transition and the stability of a hypersonic boundary layer. A numerical investigation of the disturbance evolution in a Mach-6 sharp cone boundary layer with local wall heating or cooling is conducted. Direct numerical simulation (DNS) is performed for the single-frequency and broadband disturbance evolution caused by random forcing. We vary the local wall temperature and the location of heating/cooling, and then use the e N method to estimate the transition onset. Our results show that local wall cooling amplifies high-frequency unstable waves while stabilizing low-frequency unstable waves, with local heating amplifying all unstable waves locally. The disturbance amplitude and second-mode peak frequency obtained by DNS agree well with the previous experimental results. Local cooling/heating has a dual effect on the stability of the hypersonic boundary layer. For local cooling, while it effectively inhibits the growth of the low-frequency unstable waves that dominate the transition downstream, it also further destabilizes the downstream flow. In addition, while upstream cooling can delay the transition, excessive cooling may promote it; local heating always slightly promotes the transition. Finally, recommendations are given for practical engineering applications based on the present results.

Heat Transfer and Flow Structure Characteristics of Regenerative Cooling in a Rectangular Channel Using Supercritical CO2

At an extremely high Mach number, the regenerative cooling of traditional kerosene cannot meet the requirement of the heat sink caused by aerodynamic heating and internal combustion in a scramjet propulsion system. As a supplement of traditional regenerative cooling, supercritical CO2 is regarded as an effective coolant in severe heating environments due to its excellent properties of heat and mass transportation. In this paper, the heat transfer and flow structure characteristics of regenerative cooling in a rectangular channel using supercritical CO2 are analyzed numerically using a validated model. The effect of heat flux magnitude, nonuniform heat flux, acceleration and buoyancy and flow pattern are considered to reveal the regenerative cooling mechanism of supercritical CO2 in the engine condition of a scramjet. The results indicate that the heat transfer deterioration phenomenon becomes obvious in the cooling channel loaded with relatively high heat flux. Compared with the cooling channels loaded with increased heat flux distribution, the maximum temperature increased for the channel loaded with decreased heat flux distributions. When larger acceleration is applied, a relatively lower wall temperature distribution and higher heat transfer coefficients are obtained. The wall temperature distribution becomes more uniform and the high-temperature region is weakened when the coolants in adjacent channels are arranged as a reversed flow pattern. Overall, the paper provides some references for the utilization of supercritical CO2 in regenerative cooling at an extremely high Mach number in a scramjet.

Heat Transfer Analysis and Structural Optimization for Spiral Channel Regenerative Cooling Thrust Chamber

There is currently a lack of efficient heat transfer analysis methodologies for spiral channel regenerative cooling that has been increasingly applied in liquid rocket engines. To figure out the heat transfer characteristics of the spiral channel regenerative cooling thrust chamber, a simple 1D method based on the traditional semi-empirical formula after correcting the flow velocity is proposed. The accuracy of this approach is verified by the 3D numerical simulation. The verified method is further used to analyze the distribution of inner wall temperature in the test case and optimize the channel’s parameters. The research shows that the maximum inner wall temperature cooled by the spiral channel is 8.5% lower than that of the straight channel under the same channel size and boundary condition, indicating that the application of the spiral channel significantly improves the cooling effect. In addition, the 1D model combined with the second-order response surface model is employed to optimize the channel width, channel height, pitch, and inner wall thickness aiming for the best cooling effect. The calculated maximum temperature of the inner wall after the structure optimization is 586.6 K, which is 29.8% lower than the initial structure before optimization.

Influence of High-temperature Environment on Cook-off Characteristics of Modular Charge

In order to study the heating process of the modular charge loaded into the chamber after the continuous firing of the large caliber gun in the high-temperature environment, a 2D transient cook-off model of the modular charge retained in the chamber was established with cook-off method. When the ambient temperature is 323K and the critical temperature of the inner wall of the gun chamber is 443K, the wall temperature distribution of the gun is solved after 2 rounds/min of continuous firing. Taking this as the initial temperature condition, the cook-off process of the modular charge in the chamber is numerically simulated, and then the cook-off-odd characteristics of the modular charge in the high-temperature environment are analyzed. The results show that after 33 rounds of continuous firing at 2 rounds/min at 323K ambient temperature, the temperature of the inner wall of the gun chamber reaches 442.7K, and the modular charge would have a cook-off response after 107.5s after loading into the chamber, and the response temperature is 483.5K. The initial response position of the modular charge is located at the inner wall of the upper right corner of the module cartridge. The consistency between numerical calculation and experimental results shows that the model could analyze the cook-off problem of modular charge in bore well.