Wall Heat Transfer Research Articles

High-precision 2D surface phosphor thermometry at kHz-rates during flame-wall interaction in narrow passages

This work demonstrates high-speed 2D wall temperature measurements occurring during flame-wall interaction (FWI) within a narrow channel. Such measurements are essential to understand transient wall heat transfer and flame behavior occurring within micro-combustors and designated engine crevices. Wall temperature is measured using the phosphor Bismuth-doped Scandium vanadate (ScVO4:Bi3+). ScVO4:Bi3+ exhibits a short phosphorescence lifetime (∼ 2 µs at room temperature), enabling kHz measurement rates. ScVO4:Bi3+ also exhibits a high temperature sensitivity, which yields single-shot precision < 0.5 K within the temperature range of 295 – 335 K. A frequency-doubled Ti:Sapphire laser emitting light at 400 nm is used to excite ScVO4:Bi3+, and wall temperature is measured within a ∼ 22 × 22 mm2 region with 380 µm spatial resolution. Phosphor thermometry and CH* imaging are combined at 1 kHz to measure the spatiotemporal dynamics of the flame and wall temperature (Twall) within a 2 mm crevice passage in a fixed volume chamber designed for heat transfer studies. Measurements describe Twall signatures associated with transient FWI events, including unique Twall features associated with wrinkled flame fronts. For our operating conditions, Twall associated with flame cusps consistently exhibit temperatures 5 – 20 K lower than flame crest regions. The most extreme difference is seen for large cusp formation, where local wall cooling is noticeable at the trough of the flame cusp. This cooling feature may be caused by intrinsic flame/flow instabilities, which locally and temporally cool the wall.

Effects of the inclination angle of oval film holes on the aero-thermal performance of a squealer blade tip

The investigation and study on the characteristics of the flow and wall heat transfer on the squealer tip is of vital importance to the safe and durable operation of a turbine rotor. The numerical model of the blade squealer tip with discrete film cooling holes (round and oval holes) was constructed and validated in this paper. The effects of the radial and axial inclination of oval-holes on aero-thermal performance were systematically investigated, which received little attentions in previous studies. The result shows that the streamline deflection produced by negative inclination structures significantly improves the coverage of film, but the development of the jet vortex leads to an enhancement in leakage flow. The lift-off of film is effectively restrained by positive inclination structures, however, the jet outflow contraction limits the expansion of film coverage. Besides, the scraping between tip cavity flow and leakage flow is also an important factor to the leakage flow control, which can reduce the momentum of leakage flow. Overall, the negative radial inclination structure achieves the optimal performance of film cooling, whose tip average cooling effectiveness at 0.5 blowing ratio is even 132% higher than that in the round hole reference at 1.5 blowing ratio. The highest relative reduction of leakage flow is obtained by the positive axial inclination structure with 62.79% at the blowing ratio of 1.5.

Hypersonic shock impingement studies on a flat plate: flow separation of laminar boundary layers

Interactions between shock waves and boundary layers produce flow separations and augmented pressure/thermal loads in hypersonic flight. This study provides details of Mach 7 impinging-shock-flat-plate experiments conducted in the T4 Stalker Tube. Measurements were taken at flow conditions of Mach 7.0 (2.44 MJ kg $^{-1}$ ) and Mach 7.7 (2.88 MJ kg $^{-1}$ ) flight enthalpies with a range of freestream unit Reynolds numbers from $1.43 \times 10^{6}$ m $^{-1}$ to $5.01 \times 10^{6}$ m $^{-1}$ . A shock generator at $12^{\circ }$ or $16^{\circ }$ to the freestream created an oblique shock which impinged on a boundary layer over a flat plate to induce flow separation. The flow field was examined using simultaneous measurements of wall static pressure, heat transfer and schlieren visualisation. Measured heat transfer along the flat plate without the shock impingement indicated that the boundary layer remained laminar for all flow conditions. The shock impingement flow field was successfully established within the facility test duration. The onset of separation was observed by a rise in wall pressure and a decrease in heat transfer at the location corresponding to the stem of the separation shock. Downstream of this initial rise, an increased pressure and higher heating loads were observed. The heat-transfer levels also indicated an immediate boundary layer transition due to the shock impingement. The separation data of the present work showed good agreement with our previous work on shock impingement on heated walls (Chang et al., J. Fluid Mech., vol. 908, 2021, pp. 1–13). A comparison with the previous scaling indicated that the separation also relates to the pressure ratio and the wall temperature parameter.

On the Coupled Thermal and Hydrodynamic Interaction of Adjacently Located Vapor Bubbles on Highly Wetting Surfaces.

Spatio-temporally resolved IR measurements coupled with high-speed videography have been made to propose the regime map of the interaction between two adjacently located vapor bubbles on high-wettability surfaces. The modes of interaction (thermal and/or hydrodynamic) have been studied as a function of distance between the two nucleation sites under saturated nucleate pool boiling conditions with water as the working fluid, and their effects on the wall heat-transfer rates have been quantified. Based on the comprehensive observations and analyses of the simultaneously mapped vapor bubble dynamics, substrate surface temperature, and the associated wall heat flux distributions, the possible regimes of bubble interactions have been identified. Experiments revealed three dominant mechanisms of bubble interactions: hydrodynamic interaction (HI), thermal interaction (TI), and horizontal coalescence (C). Depending on the relative spacing of the two nucleation sites, a regime map has been prepared wherein various possible bubble interactions (and/or their combinations) have been classified as HI + TI + C, HI + C, and HI. The IR thermography-based measurements revealed a strong dependence of bubble base evaporation-driven heat transfer on the possible bubble interaction mechanism(s) encountered in each regime. The dependence of microlayer formation beneath the growing vapor bubble(s) and its evaporation on HI is further corroborated through thin-film interferometric measurements. Plausible explanation(s) for mechanisms through which bubble interactions influence heat transfer have been discussed. A trend of variation of wall heat-transfer performance with bubble interaction regimes has been deduced and discussed.

Experimental Research on the Flow and Heat Transfer Characteristics of Subcritical and Supercritical Water in the Vertical Upward Smooth and Rifled Tubes

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%.

Impact of Irregular Heat Sink/Source on the Wall Jet Flow and Heat Transfer in a Porous Medium Induced by a Nanofluid with Slip and Buoyancy Effects

In many industries, extremely high-performance cooling is a crucial requirement. However, the fundamental challenge to developing energy-efficient heat transfer fluids required for cooling is insufficient thermal conductivity. In this case, the utilization of nanofluid is effective to overcome these challenges. The current study aims to examine the two-dimensional (2D) stretching wall jet heat transfer fluid flow induced by a water-based alumina nanofluid embedded in a porous medium with buoyancy force. In addition, irregular heat sink/source and slip effects are assessed. The leading partial differential equations are changed into ordinary differential equations by incorporating similarity variables, then these equations are computationally or numerically worked out via the boundary-value problem of fourth-order (bvp4c) technique. The pertinent factors influencing the symmetry of the hydrothermal performance including friction factor, velocity, and temperature profiles, are illustrated using tables and graphs. The symmetrical outcomes reveal that the velocity declines in the presence of nanoparticles, whereas the temperature uplifts both assisting and opposing flows. Moreover, the friction factor augments due to porosity while the heat transfer rate declines.

Assessment of the Flow Field and Heat Transfer in a Vane Cooling System Using Magnetic Resonance Velocimetry, Thermochromic Liquid Crystals, and Computational Fluid Dynamics

AbstractComputational fluid dynamics (CFD) is the standard tool in the turbomachinery industry to analyze and optimize internal cooling systems of turbine components, but the code applied has to be validated. This paper presents a combined experimental and numerical study on the flow field and heat transfer in a cooling system consisting of a three-pass serpentine with rib turbulators and trailing edge ejection. The cooling geometry is taken from a stator vane currently used in an industrial gas turbine and operates at a coolant inlet Reynolds number of 45,000. As an experimental technique, magnetic resonance velocimetry (MRV) was used to obtain the three-dimensional time-averaged velocity field of the isothermal flow. The measurements were conducted in a large-scale model and resulted in 3.2 million velocity vectors and measurement uncertainty of 6.1% of the bulk inlet velocity. The local wall heat transfer was measured in a separate experiment using thermochromic liquid crystals (TLC). These measurements yielded the distribution of the heat transfer coefficient on both the pressure and the suction side internal walls with a measurement uncertainty of 12%. The experimental data are used as a reference for the numerical study. In total, eight turbulence models are evaluated here, including one-equation, two-equation, algebraic and differential Reynolds stress models, and a scale adaptive simulation. The results show the differences between the velocity fields and the heat transfer coefficient distribution, allowing for the identification of the optimum turbulence model for this particular type of flow.

Compressible modification of boundary-layer momentum thickness localized method in transition model

In current transition models, Menter’s correlation between the maximum vorticity Reynolds number Revmax in the boundary layer and momentum thickness Reynolds number Reθ has been widely used in the localization of the boundary layer length scale. However, the existing correlations have problems in correctly reflecting compressibility effects and only relying on local variables. In this paper, the similarity solutions of laminar compressible boundary layer equations are adopted to study the relation between Revmax and Reθ. Through extracting the local Mach number Malocal and the ratio of wall temperature to the local temperature Tw/Tlocal right at the position where Revmax happens, the influences of the Mach number effect and wall heat transfer effect are characterized, meanwhile a new correlation only relying on local variables between Revmax and Reθ is proposed. For plat-plate boundary layers, the relative error is tiny within a large Mach number and wall-to-adiabatic temperature ratio range and the error caused by the streamwise pressure gradient is not of great concern for general pressure gradients. Furthermore, numerical results over common geometries, including wedges, sharp cones and blunt cones, demonstrate that the proposed correlation could give much better predictions of the relation between Revmax and Reθ than previous correlations. At last, the present correlation is applied to the γ-Reθ transition model as an application presentation.

Wall heat transfer and flame structure transitions in stagnating spray flames

Wall-stagnating spray flames are of importance in a number of engineering applications, including accidental leakage of liquid fuel from pressurized fuel lines as well as in direct-injected compression and spark-ignition engines. The wall heat flux generated by such flames is an important consideration in the design and analysis of affected components. To identify and analyze the key parametric dependencies of the flame structure and wall heat flux, we carry out one-dimensional simulations of wall-stagnating spray flames employing an Eulerian-Eulerian formulation and a realistic 54-species chemical mechanism for n-dodecane/air combustion. Conjugate heat transfer is accounted for by considering a finite-thickness wall. We find that the configuration permits three distinct flame structures, namely wall-stabilized, detached, and injector-stabilized flames. Since the near-wall flame structure determines the wall heat flux, the parameter space is explored to identify points of flame structure transition. We show that the Stokes number and the liquid mass loading play a key role in controlling the transition in the flame structure and its non-linear coupling to the wall heat flux, the former parameter by controlling the droplet evaporation time and the latter by affecting the amount of fuel able to penetrate the flame. The wall boundary temperature has a direct effect on the wall heat flux, and we show that it exhibits hysteresis in flame structure transition, with an associated change in wall heat flux of up to 30 percent.

Simulation and optimization of the thermal behavior of a Trombe wall under unsteady conditions

This paper aims to analyze the heat transfer in an unvented Trombe wall. The wall exchanges by convection and radiation with two internal and external environments. During winter, it absorbs a variable solar flux through its outer surface, while during summer, it absorbs solar radiation via the glazing thanks to a thin black film. The conservation equations governing the problem were discretized by the control volume method and solved by the SIMPLE algorithm. The radiosity method was applied to evaluate the radiative fluxes exchanged by the lateral surfaces of the air layer. These numerical approaches were implemented in a numerical solver, which is validated against the findings of a similar study. Indeed, the differences recorded between the results do not exceed 7 %. In this work, the authors studied the thermal performance of the Trombe wall considering different building materials (fired clay, sand-lime, concrete, and fly ash) and different working fluids (air, helium, neon, argon, krypton, and xenon). The results are presented in terms of hourly variations of the incoming and outgoing fluxes and some metrics measuring the thermal performance of the studied system.

Two-way coupled turbulent particle-laden boundary layer combustion over a flat plate

In the present study, turbulent particle-laden boundary layer combustion over a flat plate is investigated using direct numerical simulation (DNS). A two-way coupled Eulerian–Lagrangian point particle method is used for the solid phase. The effects of particle Stokes numbers, mass loadings and chemical reactions on the interactions between particles and boundary layer turbulence in the near-wall region are explored. It was found that particle heat transfer is dominant over wall heat transfer in the reacting case with heavy particles and large mass loadings, resulting in a lower fluid temperature. Particle accumulation due to the turbophoresis effect in the near-wall region is observed, which is more prominent in the cases with a large Stokes number. The turbophoresis effect is examined via the magnitude of streamwise vorticity $\varOmega _x$ . It is shown that $\varOmega _x$ is attenuated by heavy particles, and the attenuation increases with increasing mass loadings. Therefore, particle wall–accumulation is less prominent in the cases with large mass loadings. Compared with the non-reacting cases, the distribution of particles is more inhomogeneous for the reacting cases, where the particles move faster due to intense reactions with increasing wall-normal distance. Finally, the flow topologies and the Reynolds stress anisotropy are examined to understand turbulence modulation by combustion and particles. It was suggested that light particles augment the vortex-dominant topologies, whereas heavy particles have an opposite effect. The anisotropy mapping of the Reynolds stress shows that the flows become more one-dimensional in the near-wall region for the cases with combustion and/or large mass loadings.

Analysis of Jet Wall Flow and Heat Transfer Conveying ZnO-SAE50 Nano Lubricants Saturated in Darcy-Brinkman Porous Medium

The problem of 2D (two-dimensional) wall jet flow, along with heat transfer incorporated by nanofluid in a Darcy-Brinkman medium, while recognizing the requirement for efficient heating and cooling systems. Following the use of similarity variables, the resultant system of ODEs (ordinary differential equations) is solved using the well-known and efficient bvp4c (boundary-value problem of the 4th order) technique. The significance of physical quantities for the under-consideration parameters is illustrated and explained. The findings show that the nanoparticle volume fraction and porosity parameters decrease the velocity, but increase the temperature. In addition, the temperature uplifts in the presence of radiation effect. The suction parameter initially decreases and then increases the velocity near the surface, while the temperature declines.

Analysis of the heat-carrying performance of natural circulation loop using modified RELAP5/MOD3.4

Since the successful manufacture of the separate heat pipe (also known as Two-phase Loop Thermosyphon or Natural Circulation Loop), it has been used in many applications, especially for waste heat recovery/remove and cooling systems. We use the modified system analysis program RELAP5 to study the medium-sized separated heat pipe setup MSL2 built by Seok-Ho Rhi. To better calculate heat pipe performance under low pressure, we have improved a series of models, including the bubble nucleation model, the heat flux partition model, the interfacial heat transfer, the drift flux model, the wall heat transfer model, etc. Then we use the low-pressure subcooled boiling experiment to verify the modified code. The validation results show that the modified code accurately captures the subcooling boiling phenomenon under low pressure and low flow conditions. We use the modified code to investigate the factors influencing heat-carrying performance and analyze their action mechanisms. The results show that heating power, volumetric filling rate, and cooling water temperature influence heat-carrying performance. Additionally, we analyze the heat pipe operating parameters over time to obtain the characteristics of the heat pipe at various stages.

Predicting mechanical properties of material extrusion additive manufacturing-fabricated structures with limited information

Mechanical properties of additively manufactured structures fabricated using material extrusion additive manufacturing are predicted through combining thermal modeling with entanglement theory and molecular dynamics approaches. A one-dimensional model of heat transfer in a single road width wall is created and validated against both thermography and mechanical testing results. Various model modifications are investigated to determine which heat transfer considerations are important to predicting properties. This approach was able to predict tear energies on reasonable scales with minimal information about the polymer. Such an approach is likely to be applicable to a wide range of amorphous and low crystallinity thermoplastics.

Coupled heat and moisture transfer in walls featuring moisture-buffering materials and ventilating layers: An Experimental study

The moisture performance of building envelopes largely depends on the building materials, construction techniques, and exposure loads from the indoor and outdoor regions. A ventilated air interlayer placed in a wall can help dehumidify the wall and indoor air. This paper presents an experimental study of the heat, air, and moisture variations within the envelope wall of a chamber featuring different air interlayer settings under real outdoor air conditions during the summer of 2020 in Shanghai, China. Self-developed humidity-controlling building materials were applied to the inner building envelope. Temperature, humidity, wind velocity, and heat-flow sensors were placed at different positions in the middle of the wall. These parameters were measured and recorded in real-time under three working conditions: humidification, dehumidification, and ventilation. The experimental results show that under the ventilation working conditions, moisture content of 0.52 kg can be removed after a 2-h air layer ventilation, which can benefit the design strategy for the humidification and ventilation of dehumidification walls.

Performance evaluation of an active pipe-embedded building envelope system to transfer solar heat gain from the south to the north external wall

The unequal heat gain in south and north orientation of a building causes higher energy consumption which should have been avoided. In this paper, an active pipe-embedded building envelope system is proposed to achieve heat redistribution between north and south rooms to reduce building heating load. The system is mainly composed of closed-loop pipes embedded in external walls which will absorb and transfer the solar heat gain from the south façade to the north. On cloudless days, the structure can effectively raise the surface temperature of the north wall, thus reducing the heat loss through the envelope. We present all the mathematical equations and a mathematical model validated against experimental data in the literature. To further check the feasibility of the system, the heat transfer of a pipe-embedded external wall and a conventional wall are compared, with the same configurations under typical weather conditions in five building climate zones of China. The results show that the heating load reduction during heating season for the room adopting this system relative to the one without it is 12.8% for hot summer and cold winter climate. For severe cold climate and cold climate, the heating loads in January are reduced by 4.6% and 8.7%, respectively. The system plays a minor role in reducing building energy consumption in the hot summer and warm winter zone. Besides, energy consumption of building in summer can also be reduced, although not as effectively as in winter. This study may guide a better design and control of low energy building in further research and practice.

Experimental and computational fluid dynamics modeling of double‐partition divided‐wall column for flow hydrodynamics and wall heat transfer characteristics

AbstractThe aim of this study is to investigate the flow hydrodynamics and wall heat transfer characteristics on the tray of a two‐staggered walled region in a double‐partition divided‐wall column (DPDWC). An experimental setup of DPDWC is designed, and a theoretical temperature gradient (TG) model based on Fourier's equation, which describes the wall heat transfer, is proposed. A coupled computational fluid dynamics‐TG model is developed to analyze the heat transfer characteristics across partitions of DPDWC. The effects of liquid velocity, gas velocity, and different wall thermal boundary conditions on the flow and temperature fields are studied. The results indicate that the lack of a gas cone near partitions led to few vertical backflows of the liquid phase on the tray. Furthermore, the TGs near the liquid phase outlet increased with an increase in the liquid phase velocity. This work provides a scope for design and operation of DPDWC for industrial implementation.

Evaporation characteristics of viscous droplets on stainless steel superhydrophobic surface

A superhydrophobic surface with a contact angle greater than 160° and a sliding angle ∼2° was fabricated by a simple, low-cost chemical etching method. The evaporation characteristics of glycerol-water droplets with different viscosities on the superhydrophobic surface and the smooth surface were investigated. The results showed that the evaporation rate on the superhydrophobic surface was smaller than that on the smooth surface for the lower concentration droplets. Droplets with higher viscosity were more prone to decrease the overall evaporation rate, and this decrease occurred earlier on smooth surfaces. As the droplet concentration increased to 80% glycerol-water, the evaporation rate was higher on superhydrophobic surfaces than that on the smooth surface. In addition, the adhesion tests were conducted and showed that the superhydrophobic surface was effective in reducing the adhesion of high viscosity liquids. Consequently, the superhydrophobic surface was conducive to enhancing the heat transfer efficiency and reducing the coking problem on the heat transfer walls.

Numerical evaluation of indoor hygrothermal behavior with heat and moisture transfer in walls by analytical conditions

Studies analyzing indoor thermal environments according to variables such as the outdoor climate, wall layer, and building composition have benn conducted using building simulations. However, previous analyses of hygrothermal performance considering moisture are insufficient. Therefore, this study propose a process for evaluating indoor hygrothermal performance according to parameters using two different simulations. First, heat and moisture transfer in walls (external and internal walls, roofs, and floors) was analyzed, and the hygrothermal performance of the indoor environment was investigated based on the hygrothermal behavior of walls. The heat and moisture transfer analysis showed that the relative humidity (RH) and water content were high inside the wall (E) and floor layers owing to regional climatic conditions. The results of the indoor environment simulation indicated a distinct distribution of temperature and humidity, owing to the heat and moisture fluxes from the inner surfaces. In addition, as a result of improving the hygrothermal performance by changing the building materials for exterior walls, the heat flux decreased by 30%, and the moisture flux decreased by 58%. These results confirmed that the boundary conditions for temperature and humidity required to evaluate indoor hygrothermal environments can be analyzed according to climatic conditions and wall layer compositions.

Experimental and numerical study on the heat transfer characteristic of supercritical water with high mass velocity in a helically coiled tube

To explore the flow and heat transfer characteristics of supercritical fluid with high mass velocity in a helically coiled tube, experiments and numerical simulations were conducted to evaluate the wall temperature distribution and heat transfer coefficient in a vertical helically coiled tube under a pressure of 24–28 MPa, with a mass velocity of 2500–4000 kg·m−2·s−1 and with a heat flux of 210–420 kW·m−2. Results show that the coil's inner wall temperature is the highest and the outer wall temperature is the lowest because the centrifugal effect is much stronger than the buoyancy effect at a high mass velocity in a helically coiled tube and thus the fluid with lower temperature and consequently higher density shifts to the outer side of the coil. To describe the wall temperature difference in the different circumferential points, a metric called the circumferential wall temperature inhomogeneity (CWTI) is proposed, and the influence of the parameters on the CWTI is then analysed. As bulk fluid temperature increases, the CWTI first decreases to a minimum at the pseudo-critical temperature and then increases thereafter. The increase of mass velocity reduces the CWTI, and the influence of mass velocity on the CWTI is the most significant in the high enthalpy region. In the high enthalpy region with the bulk fluid temperature of 410.1 °C, when the mass velocity increases from 2500 to 3250 kg·m−2·s−1, the CWTI decreases by 34.2%. The CWTI also increases almost linearly with the increase in heat flux. Finally, the factors influencing the average heat transfer coefficient in the helical tube are discussed. The coefficient first increases and then decreases with increase in the bulk fluid temperature, and the peak value of the coefficient appears at the pseudo-critical point corresponding to the supercritical pressure. Increasing the mass velocity improves the average heat transfer coefficient, and the improvement effect is more obvious after the pseudo-critical point. Increasing the pressure causes the bulk fluid temperature at which the peak average heat transfer coefficient is achieved to increase, and the peak value gradually decreases. When the pressure increases from 24.0 MPa to 26.0 MPa and to 28.0 MPa, the peak value of the average heat transfer coefficient decreases by 34.2% and 46.4%, respectively.