Primary Heat Transfer Mechanism Research Articles

Numerical study of heat transfer characteristics of phase change materials reinforced with porous media based on hybrid mesh

To address the drawbacks of inadequate energy density as well as utilization effectiveness in solar energy utilization, phase change energy storage offers a possible option. However, the low thermal conductivity of traditional phase change materials (PCMs) restricts the extent to which solar energy utilization can be enhanced. In this paper, the impact of the metal foam (MF) on the solid–liquid interface, the flow field, the enhancement heat transfer mechanism during the paraffin wax (PX) melting process and thermal storage characteristics was analyzed. Besides, a three-dimensional mathematical model of MF-PX using the hybrid mesh technique was established and software ANSYS-FLUENT was used for simulation. The results indicated that, the hybrid mesh exhibited a reduction of 52.75 % in mesh count compared to the unstructured mesh, resulting in 45 % reduction in computational time. Moreover, the hybrid mesh also showcased improved mesh quality that the unstructured mesh was mainly distributed in the range of 0.48–0.6, while within the range of 0.84–1.0 for the hybrid mesh. More important, the maximum relative error between experimental and simulation was 8.67 %, indicating that the model can make a reasonable prediction. With the proportions of MF increased from 0.11 % to 0.43 %, both the Rayleigh number (Ra) and its amplification decreased. At 800 s, the Ra decreased from 1.76E+07 to 1.20E+07, while the amplification of the Ra was reduced from 1.88E+07 to 1.30E+07. In addition, the natural convection proportions of CMF decreased from 96.31 % to 80.65 %. These observations indicated that natural convection was the primary heat transfer mechanism, and the conduction was enhanced by the increasing proportion of MF. Finally, in terms of heat storage characteristics, the heat storage capacity and heat storage rate of the composite PCM slightly decreased. The heat storage values decreased from 19.30 kJ to 18.66 kJ. Similarly, the heat storage rates decreased from 19.86 J/s to 18.81 J/s.

Research on the thermal behavior of medium-temperature phase change materials and factors influencing the thermal performance based on a vertical shell-and-tube latent heat storage system

Medium-and-high temperature latent heat storage systems have been developed to address challenges related to exhaust heat utilization and energy peak regulation in the field of energy engineering. However, there is still a lack of understanding regarding the thermal behavior of medium-and-high temperature phase change materials within heat exchangers and the factors influencing the system's thermal characteristics. Therefore, this study designed and constructed a new medium-temperature (300 °C) latent heat storage system with spiral-finned heat exchanger tubes. It uses 260 kg of medium-temperature phase change material for 68.3 MJ energy storage over 2 h. The system's response to variations in the inlet temperature and flow rate of the heat transfer fluid is investigated using thermocouples and visualization windows and provides recommendations for operating parameters. Results show natural convection as the primary heat transfer mechanism in the charging process, leading to temperature stratification in the tank (the upper has a higher temperature than the lower part). In contrast, heat conduction dominates heat transfer during discharging, resulting in a more uniform temperature distribution in vertical latent heat storage systems. Besides, inlet parameters significantly influence the system. Increasing storage temperature by 20 °C or flow rate by 20 L/min reduces storage time by 33.5 % and 54.0 %, respectively. In contrast, during the exothermic stage, heat conduction dominates with a more uniform temperature distribution. Optimal conditions are found at 280–150 °C, 15 L/min, achieving an input of 84 MJ and output of 50.25 MJ over 200 min, with a cycle efficiency of approximately 59.82 %.

Coupling radiative properties with detailed char conversion kinetics

In pulverized solid fuel combustion, radiation is the primary heat transfer mechanism. Especially in the near-burner region with high particle loadings, particle radiation dominates gas radiation. Thus, adequate modeling of particle-radiation interactions in full-scale simulations of combustion chambers may primarily affect the overall accuracy. The radiation behavior depends strongly on the particles? optical properties, characterized by the complex index of refraction (IOR). The IOR depends on particle morphology and material composition which change during pyrolysis and char conversion. For char conversion, the focus of the present paper, the change depends on how the structural transformations take place inside the particle, characterizes by the reaction regimes, boundary layer diffusion limited, pore diffusion limited or kinetically controlled. The formation of an outer ash film layer with progressing burnout can complicate the situation even more.To quantify the effects of the structural changes during the char conversion process on the radiative properties, the output data of the char conversion kinetics (CCK) model (particle size, ash/carbon content, pore fraction) are used to build artificial particles at different conversion degrees and for the different regimes. Finally, the radiation interaction is calculated by applying an extension of the Mie theory for coated particles, and these results are compared to simplified approaches.Simplified approaches were tested to derive the IOR by applying mixing rules based on the available char and ash content with progressing char conversion. The results indicate that only a non-linear mixing rule for the IORs of ash and char lead to appropriate approximations of the radiation behavior of the particle. Furthermore, it is not necessary to model the spatial arrangement of carbon core and ash film for the radiative properties. Modeling a sphere and calculating an effective IOR depending on the carbon, ash, and pore volume fraction results in an average deviation of less than 3% to the exact solution.

Steady melting in material extrusion additive manufacturing

PurposeA main cause of defects within material extrusion (MatEx) additive manufacturing is the nonisothermal condition in the hot end, which causes inconsistent extrusion and polymer welding. This paper aims to validate a custom hot end design intended to heat the thermoplastic to form a melt prior to the nozzle and to reduce variability in melt temperature. A full 3D temperature verification methodology for hot ends is also presented.Design/methodology/approachInfrared (IR) thermography of steady-state extrusion for varying volumetric flow rates, hot end temperature setpoints and nozzle orifice diameters provides data for model validation. A finite-element model is used to predict the temperature of the extrudate. Model tuning demonstrates the effects of different model assumptions on the simulated melt temperature.FindingsThe experimental results show that the measured temperature and variance are functions of volumetric flow rate, temperature setpoint and the nozzle orifice diameter. Convection to the surrounding air is a primary heat transfer mechanism. The custom hot end brings the melt to its setpoint temperature prior to entering the nozzle.Originality/valueThis work provides a full set of steady-state IR thermography data for various parameter settings. It also provides insight into the performance of a custom hot end designed to improve the robustness of melting in MatEx. Finally, it proposes a strategy for modeling such systems that incorporates the metal components and the air around the system.

Experimental study on the thermal characteristics of a visualized shell-and-tube LHTES system at different endothermic and exothermic temperatures

This research developed a novel medium-temperature (up to 300 °C) latent heat thermal energy storage (LHTES) system, utilizing a compact shell-and-tube thermal exchange system containing spiral finned tubes, 220 kg of phase change material (PCM) was employed, enabling 44.52 MJ of heat storage, a visible window allowed to observe the phase change process and investigate the temperature variations in conjunction with thermocouples. The study analyzed the thermal traits and explored the impact of charging and discharging temperatures on the system's thermal performance.The outcomes indicated that natural convection gradually supplants heat conduction in the charging process as the primary heat transfer mechanism and a temperature stratification phenomenon emerges within the TES; during the discharging process, a contrasting behavior is observed. Employing charging-discharging temperature configurations of 290-170, 280-160, and 270-150 °C, the cumulative energy input to complete the cycle process is 63.59, 56.43, and 54.87 MJ, correspondingly, the released energy is 33.64, 33.41, and 33.57 MJ, yielding cycle efficiencies of 52.90, 59.20, and 61.20 %, respectively, with the charging temperature rising from 270 to 290 °C, storage time decreased 25.6 %, but lead to an 8.49 % decrease in heat storage efficiency, while the discharging temperature has a relatively minor influence on the system.

A novel nonlinear radiative heat exchanger for molten-salt applications

One of the main difficulties associated with using molten salts as heat transfer fluids (HTFs) in practical applications such as in the solar and nuclear energy industries is the possibility of solidifying molten salts. Since the salts proposed for Generation-IV nuclear reactors (MSRs) have higher melting points, this is especially true for those reactors. The consequences of accidental freezing within the direct reactor auxiliary cooling system (DRACS) of MSRs could be catastrophic. DRACS is a passive safety system designed to remove decay heat from the reactor core during an emergency. The present study examines, as a case study, the freezing problem in the most vulnerable component of DRACS, namely the natural draft heat exchanger (NDHX). For this application, a unique and innovative heat exchanger design termed radiative heat exchanger (RHX) is proposed, with nonlinear heat transfer characteristics. Specifically, by suppressing convection and exploiting thermal radiation as the primary heat transfer mechanism, the RHX concept reduces the heat rejection/removal rate passively to allow safe operation when the molten salt coolant temperature drops and is near its melting point, while maintaining a high capacity for decay heat extraction in case the coolant temperature rises during an accident and the reactor becomes overheated; this moderation of the heat removal rate is accomplished without the requirement for external (manual or automatic) control. A thermohydraulic model is developed and applied to evaluate the proposed RHX with the existing conventional NDHX. Compared to the conventional design, the RHX has the potential to reduce heat removal by approximately 30 % at low temperatures and near-solidification conditions. In the case of the prototypical 20 MW reactor with a decay rate of 0.2 MW, RHX extends the freezing time of molten salt by three times, to approximately 18 h. During high-temperature events, such as the initial stages of a reactor scram, the RHX can improve decay heat extraction by 17 %. Other applications of molten salts with similar challenges can also be addressed using the proposed design approach.

A Miniaturized, Green, End-Burning, and Sandwich Hybrid Propulsion System

Conventional hybrid rocket motors with thrust levels greater than 5 N rely on forced convection within the boundary layer as the primary heat transfer mechanism for fuel vaporization. For hybrid rockets with thrust levels of less than 5 N, oxidizer mass flow levels are sufficiently small that the rate of convective heat transfer is significantly reduced; and radiative heat transfer dominates the fuel vaporization mechanism. Radiative heating is a concern when implementing traditional hybrid rocket core-burn fuel grain designs for systems with thrust levels suitable for small satellites and CubeSats. Radiative heating induces a fuel-rich burn, which leads to low combustion efficiency and nozzle clogging. This paper presents a novel idea of using radiative heating in the design of a hybrid propulsion system suitable for CubeSats and small satellites. Additionally, this paper presents the test results of two fuel grain designs: an end-burning fuel grain design and a “sandwich” fuel grain design. This paper presents the test results of a 0.5 N end-burn system that produces a vacuum of 162 s with a rise time of 1.1 s and a minimum impulse bit of . This paper also presents the test results of a 1.0 N end-burn system that produces a vacuum of 136 s with a rise time of 2.2 s and a minimum impulse bit of . In addition, the test results for a 1 N sandwich system that produces a vacuum of 170 s with a rise time of 0.3 s and a minimum impulse bit of are presented. Acrylonitrile butadiene styrene, polyvinyl chloride, Nylon-12, and an acrylic plastic known as polymethyl methacrylate were used as fuel; and gaseous oxygen was used as the oxidizer during these testing campaigns. These propellants provide several advantages, including benign handling properties, simplified plumbing, and greater burn efficiency over traditional monopropellant hydrazine.

Experimental analysis of the heat transfer rate of phase change material inside a horizontal cylindrical latent heat energy storage system

Latent heat thermal energy storage system can help in the smooth operation of energy supply and demand. In the present work, experimental study was performed to analyze the heat transfer rate of phase change material inside an annulus. A copper pipe (centrally loaded) runs the length of a cylindrical container during both discharging and charging modes. The overall heat transfer rate is enhanced during phase change process by adding longitudinal fins with the copper pipe. These longitudinal fins were tilted at an angle of 120⁰. Paraffin wax is used as phase change material. The main objective of the experiment is to analyze the heat transfer rate during the solidification and liquefaction of PCM. Also, the effect of mass flow rate and inlet temperature of heat transfer fluid was analyzed. Conduction was found to be the primary heat transfer mechanism during the initial stages of charging. Once the PCM get liquefied inside the system, natural convection gets dominated. During the solidification of PCM conduction heat transfer get dominated. Also, it was observed that melting duration is strongly affected by the inlet temperature of HTF while the flow rate slightly affects the melting duration.

Analytical Model Quantifying the Net Coolant Flow Rate to a Microstructured Copper Surface Validated With Two-Phase Particle Image Velocimetry Measurements

Abstract Using particle image velocimetry (PIV), the amount of fluid required to sustain nucleate boiling was quantified to a microstructured copper circular disk. Having prepared the disk with preferential nucleation sites, an analytical model of the net coolant flow rate requirements to a single site has been produced and validated against experimental data. The model assumes that there are three primary phenomena contributing to the coolant flow rate requirements at the boiling surface; radial growth of vapor throughout incipience to departure, bubble rise, and natural convection around the periphery. The total mass flow rate is the sum of these contributing portions. The model accurately predicts the quenching fluid flow rate at low and high heat fluxes with 4% and 30% error of the measured value, respectively. For the microstructured surface examined in this study, coolant flow rate requirements ranged from 0.1 to 0.16 kg/s for a range of heat fluxes from 5.5 to 11.0 W/cm2. Under subcooled conditions, the coolant flow rate requirements plummeted to a nearly negligible value due to domination of transient conduction as the primary heat transfer mechanism at the liquid/vapor/surface interface. PIV and the validated analytical model could be used as a test standard where the amount of coolant the surface needs in relation to its heat transfer coefficient or thermal resistance is a benchmark for the efficacy of a standard surface or boiling enhancement coating/surface structure.

Study on bifurcation to chaos of surface tension gradient driven flow

As the primary heat and mass transfer mechanism in space through natural convection, surface tension gradient-driven convection is a complex nonlinear process concerning strong coupling between fluid flow and heat transfer. It is also a multiple parameter coupling process that exhibits complex spatial-temporal characteristics. Therefore, the mechanism of the surface tension gradient-driven convection becomes a hotspot in microgravity fluid physics. It also has many important applications, such as in space fluid and energy management. In this paper, recent experimental and numerical results on the transition of surface tension gradient-driven convection are reviewed, especially the nonlinear analysis on the flow bifurcations to chaos. There are several numerical methods to obtain the corresponding bifurcation diagrams. One is to integrate the model forward in time starting from different parameters and initial values, and others are to calculate the asymptotic flow states and bifurcation points directly. The direct numerical simulation method and time series analysis are widely used, but searching for bifurcation points from a large number of data is burdensome. Bifurcation points can be computed directly with the numerical bifurcation method, but such calculations are more difficult to implement than the direct numerical method.

Experimental Investigation Into the Heat Transfer Mechanism of Oscillating Heat Pipes Using Temperature Sensitive Paint

Abstract Oscillating heat pipes (OHPs) represent a promising passive mechanism for the removal or spreading of heat. While simple to construct, the fluid and thermodynamics of these devices are still poorly understood. There is debate over whether the primary heat transfer mechanism is due to sensible heating of the liquid phase or due to latent heat transfer through phase change. To answer this question, an experimental apparatus was constructed to provide time- and space-resolved temperature and heat transfer data across the face of an operating OHP with HFE-7000 as the working fluid. This experiment utilized temperature sensitive paint (TSP) alongside visual recording of the fluid motion in order to determine the relative latent and sensible contribution to the overall heat transfer. The OHP was tested with input powers ranging from 2.6 W to 10.1 W. It found that latent heat transfer was the dominant heat transfer mechanism, accounting for between 65% and 83% of the total heat transferred in all cases.

Effect of the heat transfer liquid fin on critical heat flux enhancement under ERVC condition

Several critical heat flux (CHF) enhancement methods for in-vessel retention through external reactor vessel cooling (IVR-ERVC) have been researched. However, they have some limitations for practical applications. In the present study, heat transfer liquid fin concept was suggested to address the issue of CHF by spreading focusing effect of the heat flux. The liquid fin was located outside of the reactor pressure vessel; it is activated by filling under the accident situations. Liquid metal and oil were selected as the fin materials as they are characterized by significantly different Prandtl numbers; this property helps to clearly distinguish the heat transfer mechanisms involved. Analysis were conducted based on both experimental and numerical methods. As expected, conduction and natural circulation were the dominant heat transfer mechanisms in the case of liquid metal fin and the oil fin, respectively. Regardless of the Prandtl number of the fin, safety margin for the CHF was significantly increased in both fin cases. The heat flux profile of the conduction dominant liquid metal fin showed a moderately flattened shaped. However, for the oil fin, heat flux was mitigated and upwardly shifted (natural circulation as the primary heat transfer mechanism). Although both materials exhibited good performance in terms of securing a margin for CHF, temperature increment by the liquid fin was much higher in the oil case because of the relative lower thermal conductivity of the latter. Thus, the use of low thermal conductivity fins requires the temperature limit to be taken into consideration.

Experimental investigation of a new thermal energy storage system using thermo-sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo-sensitive magnetic fluid (MF) in a porous medium is proposed to store low-temperature thermal energy. In order to have a better understanding about the fluid flow and heat-transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat-transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat-transfer mechanism in the porous carbon TES. In practical applications in small-scale systems, the 90-PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.

Identification of nucleate boiling as the dominant heat transfer mechanism during confined two-phase jet impingement

Thermal management of high-power electronics requires cooling strategies capable of dissipating high heat fluxes while maintaining the device at low operating temperatures. Two-phase jet impingement offers a compact cooling technology capable of meeting these requirements at a low pressure drop. Generally, confined impingement geometries are used in electronics cooling applications, where the flow is constrained between the hot surface and orifice plate. Understanding the primary heat transfer mechanisms occurring as boiling takes place on the surface during jet impingement is important, specifically under such confined conditions. In this study, heat transfer from a copper surface is experimentally characterized in both confined jet impingement and pool boiling configurations. The dielectric liquid HFE-7100 is used as the working fluid. For the jet impingement configuration, the jet issues through a single 2 mm-diameter orifice, at jet exit velocities of 1, 3, 6, and 9 m/s, into a confinement gap with a spacing of 3 jet diameters between the orifice and heat source. Additional orifice-to-target spacings of 0.5, 1, and 10 jet diameters are tested at the lowest (Vj = 1 m/s) and highest (Vj = 9 m/s) jet velocities. By incrementing the heat flux applied to the surface and observing the steady-state response at each flux, the single-phase and two-phase heat transfer performance is characterized; all experiments were carried through to critical heat flux conditions. The jet impingement data in the fully boiling regime either directly overlap the pool boiling data, or coincide with an extension of the trend in pool boiling data beyond the pool boiling critical heat flux limit. This result confirms that nucleate boiling is the dominant heat transfer mechanism in the fully boiling regime in confined jet impingement; the convective effects of the jet play a negligible role over the wide range of parameters considered here. While the presence of the jet does not enhance the boiling heat transfer coefficient, the jet does greatly increase single-phase heat transfer performance and extends the critical heat flux limit. Critical heat flux displays a linear dependence on jet velocity while remaining insensitive to changes in the orifice-to-target spacing.

A Novel Semi-Empirical Correlation for Wall Heat Transfer During Saturated Nucleate Flow Boiling

Most of the existing empirical correlations for wall heat transfer during flow boiling show a limited predictability stemming mainly from so-called the suppression and enhancement factors, which are introduced to describe the boiling heat transfer hypothetically by a combination of nucleate pool boiling and single-phase forced convection. There is no physical basis strongly supporting the determination of these factors. This study, to avoid such limitations, presents a distinctive approach to the modeling of wall boiling heat transfer utilizing the physical concept of wall heat flux partitioning. A new correlation of local boiling heat transfer coefficient is composed of primary heat transfer mechanisms of transient conduction and forced convection. Heat transfer areas of these mechanisms replace the suppression and enhancement factors in the new correlation and are determined empirically by dimensionless analysis. Based on an experimental database of 3187 points collected for saturated boiling of various working fluids flowing inside channels of different configurations, the new correlation is obtained and compared with existing correlations widely used. The evaluation highlights much better predictability of the present correlation. While the other correlations show relatively large scattering with over 30% deviation from the experimental data, the newly proposed correlation shows an excellent agreement with a deviation of less than 10%. The good predictability would be from the well-structured physical basis and make the new correlation promising in practical boiling heat transfer analysis.

Multiphase computational fluid dynamics–conjugate heat transfer for spray cooling in the non-boiling regime

A numerical study is described to predict, in the non-boiling regime, the heat transfer from a circular flat surface cooled by a full-cone spray of water at atmospheric pressure. Simulations based on coupled Computational Fluid Dynamics and Conjugate Heat Transfer are used to predict the detailed features of the fluid flow and heat transfer for three different spray conditions involving three mass fluxes between 3.5 and 9.43 kg/m2s corresponding to spray Reynolds numbers between 82 and 220, based on a 20 mm diameter target surface. A two-phase Lagrange-Eulerian modelling approach is adopted to resolve the spray-film flow dynamics. Simultaneous evaporation and condensation within the fluid film is modelled by solving the mass conservation equation at the film-continuum interface. Predicted heat transfer coefficients on the cooled surface are compared with published experimental data showing good agreement. The spray mass flux is confirmed to be the dominant factor for heat transfer in spray cooling, where single-phase convection within the thin fluid film on the flat surface is identified as the primary heat transfer mechanism. This enhancement of heat transfer, via single-phase convection, is identified to be the result of the discrete random nature of the droplets disrupting the surface thin film.

A CFD based modelling approach for predicting steam condensation in the presence of non-condensable gases

A typical post Loss-of-Coolant-Accident (LOCA) situation in nuclear power plant containments is a major safety concern. The simultaneous presence of steam, air and potentially hazardous hydrogen, which may get generated due to the metal-water vapour reactions, can lead to a singularly dangerous situation that may jeopardize the integrity of the containment structure. After steam leakage, wall condensation inside the containment is the primary heat transfer mechanism to maintain favourable safe pressure and temperature. Condensation of steam leads to strong, local, natural convective fluid flow currents, which are coupled with buoyancy and diffusional flow of the involved species. In this paper, an attempt has been made to address these complex transport processes, involving multicomponent gas species and phase-change phenomena, by CFD approach using Ansys CFX. A Wall Condensation Model (WCM) is utilized to handle wall condensation wherein the condensate is computed as a sink term. The simulated results are benchmarked against available data from several small and large scale experiments (such as TOSQAN, CONAN and others). These validation cases are chosen so as to cover different types of geometries. Specific experiments are also performed for studying the diffusion of helium jets into air filled volumes – such situations are also validated by solving the coupled conservation equations for the multicomponent mixture. Unless the net non-condensable gas (NCG) concentration is very low (less than about 6%), it is shown that the WCM, in tandem with multicomponent mixture model (MCM), works quite well for simulating the considered containment thermal-hydraulic situations. This provides the necessary confidence to further develop and validate the CFD based approach to, in the end, be able to reliably simulate post-accident thermal-hydraulics of large containment structures.

Experimental study of turbulent natural-convective condensation on a vertical wall with smooth and wavy film interface

Pressurized water reactor nuclear plants, currently under construction, have been designed with passive containment cooling systems. Turbulent, natural-convective condensation, with high non-condensable mass fraction, on the walls of the containment vessel is a primary heat transfer mechanism in these new plant designs. A number of studies have been completed over the past two decades to justify use of the heat and mass transfer analogy for this scenario. These studies are founded upon natural-convective heat transfer correlations and, in many cases, apply a diffusion layer model to couple heat and mass transfer. Reasonable success in predicting experimental trends has been achieved when correction factors are applied. The corrections are attributed to mass transfer suction, film waviness or mist formation even though little experimental evidence exists to justify these claims. The purpose of this work is to investigate the influence of film waviness and mass transfer suction on the turbulent, natural-convective condensing flow with high non-condensable mass fraction. Testing was conducted using 0.457m×2.13m and a 0.914m×2.13m condensing surfaces suspended in a large pressure vessel. The test facility implements relatively high accuracy calorimetric and condensate mass flow measurements to validate the measured heat and mass transfer rates. Test results show a significant enhancement in heat transfer is caused by disruption of the gas boundary layer due to film waviness. This result has implications for the scalability of existing correlations. A new correlation is proposed and results compared to several datasets.

A convective model for laboratory fires with well-ordered vertically-oriented fuel beds

Several studies in the literature explore the connection between rate of spread (ROS) and wind in wildland fires. These studies show very different positions about the role of radiation and convection as heat transfer mechanisms. In the case when the fuel bed is well-ordered and vertically-oriented, there seems to be a consensus leading to suggest that convective heating is the dominant heat transfer mode in that case. The purpose of this work is to propose a convective semi-physical model for the behaviour of the rate of spread in wind, when the fuel bed is vertically-oriented. Due to a specific fuel bed arrangement, flame radiation –i.e. radiation from the part of the flame above the vegetal stratum– is neglected. Only horizontal radiation from the fuel burning particles area and convective heating are taken into account. Convective heat transfer is assumed to be the primary heat transfer mechanism. The proposed model is confronted to 172 laboratory fires with a wide range of fuel characteristics. The predicted results are also compared with two simplified models from the literature. Statistical tools are used to check the agreement between the predicted ROS and the observed one where a strong agreement is generally observed, irrespective of fuel bed characteristics.

Experimental and numerical investigation of HyperVapotron heat transfer

The divertor first wall and neutral beam injection (NBI) components of tokamak devices require high heat flux removal up to 20–30 MW m−2 for future fusion reactors. The water cooled HyperVapotron (HV) structure, which relies on internal grooves or fins and boiling heat transfer to maximize the heat transfer capability, is the most promising candidate. The HV devices, that are able to transfer large amounts of heat (1–20 MW m−2) efficiently, have therefore been developed specifically for this application. Until recently, there have been few attempts to observe the detailed bubble characteristics and vortex evolvement of coolant flowing inside their various parts and understand of the internal two-phase complex heat transfer mechanism behind the vapotron effect. This research builds the experimental facilities of HyperVapotron Loop-I (HVL-I) and Pressure Water HyperVapotron Loop-II (PWHL-II) to implement the subcooled boiling principle experiment in terms of typical flow parameters, geometrical parameters of test section and surface heat flux, which are similar to those of the ITER-like first wall and NBI components (EAST and MAST). The multiphase flow and heat transfer phenomena on the surface of grooves and triangular fins when the subcooled water flowed through were observed and measured with the planar laser induced fluorescence (PLIF) and high-speed photography (HSP) techniques. Particle image velocimetry (PIV) was selected to reveal vortex formation, the flow structure that promotes the vapotron effect during subcooled boiling. The coolant flow data for contributing to the understanding of the vapotron phenomenon and the assessment of how the design and operational conditions that might affect the thermal performance of the devices were collected and analysed. The subcooled flow boiling model and methods of HV heat transfer adopted in the considered computational fluid dynamics (CFD) code were evaluated by comparing the calculated wall temperatures with the experimentally measured values. It was discovered that the bubble and vortex characteristics in the HV are clearly heavily dependent on the internal geometry, flow conditions and input heat flux. The evaporation latent heat is the primary heat transfer mechanism of HV flow under the condition of high heat flux, and the heat transfer through convection is very limited. The percentage of wall heat flux going into vapour production is almost 70%. These relationships between the flow phenomena and thermal performance of the HV device are essential to study the mechanisms for the flow structure alterations for design optimization and improvements of the ITER-like devices' water cooling structure and plasma facing components for future fusion reactors.