NUMERICAL ANALYSIS OF ULTRAFAST HEAT TRANSFER WITH PHASE CHANGE IN A MATERIAL IRRADIATED BY AN ULTRASHORT PULSED LASER

This study is concerned with problems of ultrafast and high heat flux heat transfer with phase change. We employ the cubic interpolated propagation (CIP) method coupled with a thermoconvective model to examine the history of large-scale phase change, that is, melting and evaporation, and the mechanisms of heat transfer as a wave. It is found that wave-type heat transfer as a shock wave with phase change can be simulated without a hyperbolic heat conduction equation by means of the CIP method. Melting and evaporation occur in the energy deposition region, and energy is transferred by a shock wave beyond an energy penetration depth. The propagation velocity is hardly damped outside the energy deposition region inside aluminum thin foil, but the peak value in density, pressure, and temperature is damped rapidly. For one of the dissipation process mechanisms, generation of thermal stress can be considered. Further, it is found that the initial velocity of shock wave generated inside the energy deposition region is different for each initial incident laser intensities, though the propagation velocity is constant beyond the energy penetration depth in spite of an initial incident laser intensity.     

Theoretical and experimental study on the heat transport in metallic nanofilms heated by ultra-short pulsed laser

In this paper the mechanism of heat transport in metallic nanofilms under ultra-short pulsed laser heating is examined theoretically and experimentally. In order to easily understand the non-equilibrium heat transport in metallic nanofilms the study of heat transport behavior is first carried out in dielectrics. The analyses indicate that there may be two kinds of wave phenomena in dielectrics subjected to a periodic surface temperature. One is the thermal wave governed by the C-V model based hyperbolic equation and the other is the diffusive wave governed by the Fourier model based parabolic equation. According to the hyperbolic two step model for non-equilibrium heat transport, such two kinds of wave phenomena can also occur simultaneously in the metallic nanofilms under pulsed laser heating, where the diffusive wave is induced by the electron temperature oscillation at the surface due to the non-equilibrium between electrons and lattices. Unlike the propagation speed of the thermal wave, the propagation speed of the diffusive wave depends not only on the medium properties but also the period of the temperature oscillation at the boundary. Hence, the propagation speed of the diffusive wave in the electron gas may be of as high as 10⁶ m s⁻¹, when the laser pulse duration is less than 1 ps. A transient thermoreflectance (TTR) system has been built to measure the transient electron temperature responses caused by the femtosecond laser heating and a pump-probe technique is used to ensure the femtosecond temporal resolution in the experiments. Different from the commonly used front heating-front detecting (FF) method for measuring the material properties, a rear heating-front detecting (RF) method is applied, so that measuring the propagation speed of heat becomes available. The non-equilibrium heat diffusion model is used to fit the measured normalized electron temperature profiles of 27.2 nm, 39.9 nm and 55.5 nm Au films. The best-fitted coupling factor G basically agrees with the theoretical value 2.3 × 10¹⁶ W m⁻³ K⁻¹. The propagation speed of the diffusive wave in the electron gas can be obtained by comparing the measured delay time of peak electron temperatures of Au films with different thicknesses. The average propagation speed of the temperature oscillation or diffusive wave in Au films for the range of thickness from 27.2 nm to 55.5 nm is equal to 8.1 × 10⁵ m s⁻¹, which is close to the value predicted by the non-equilibrium heat diffusion model.     

NON-FOURIER HEAT CONDUCTION PHENOMENA IN POROUS MATERIAL HEATED BY MICROSECOND LASER PULSE

Experiments on porous material heated by a microsecond laser pulse and the corresponding theoretical analysis are carried out. Some non-Fourier heat conduction phenomena are observed in the experimental sample. The experimental results indicate that only if the thermal disturbance is strong enough (i.e., the pulse duration is short enough and the pulse heat flux is great enough) is it possible to observe apparent non-Fourier heat conduction phenomenon in the sample, and evident non-Fourier heat conduction phenomenon can only exist in a very limited region around the thermal disturbance position. The hyperbolic heat conduction (HHC) equation and the dual-phase lag (DPL) model are employed, respectively, to describe the non-Fourier heat condution process happening in the experimental sample, and the finite-difference method (FDM) is used to solve them numerically. The numerical solutions show that both the HHC equation and the DPL model can predict the non-Fourier heat conduction phenomenon emerging in the experimental sample qualitatively. Moreover, if τ_q and τ_T are assumed to have suitable values, the theoretical result of the DPL model is more agreeable to the experimental result.     

RADIATION MODELING OF STATIONARY FUSED SILICA RODS AND FIBERS HEATED BY CO2 LASER IRRADIATION

This study presents a heat transfer model for a stationary fused silica rod heated by a CO₂ laser. During laser heating, the effect of fused silica being modeled to be opaque or semitransparent to laser irradiation is studied. The radiative heat transfer caused by the emission of fused silica is modeled using the zonal method, and compared to the Rosseland diffusion approximation. The spectral dependence of the fused silica absorption coefficient in semitransparent wavelengths is approximated by a two-band model. The weighted-sum-of-gray-gas (WSGG) method is used to calculate the radiative source term. The governing equation with conduction and radiation heat transfer is solved by the finite-volume method. The importance of modeling the effects of laser energy penetration below the fused silica surface during heating, especially for small diameter fibers, is discussed. The importance of radiative heat transfer in fused silica is also discussed. Around 25 K in temperature difference is observed when the diffusion approximation is used in place of the zonal method to model the radiative transfer in fused silica.     

Convective boiling heat transfer characteristics of CO2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO₂ are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO₂ from 200 to 400 kg/m² s, heat flux from 10 to 20 kW/m², while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO₂ is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m² and a mass flux of 300 kg/m² s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.     

Bio-heat transfer analysis during short pulse laser irradiation of tissues

The objective of this paper is to analyze the temperature distributions and heat affected zone in skin tissue medium when irradiated with either a collimated or a focused laser beam from a short pulse laser source. Experiments are performed on multi-layer tissue phantoms simulating skin tissue with embedded inhomogeneities simulating subsurface tumors and as well as on freshly excised mouse skin tissue samples. Two types of lasers have been used in this study – namely a Q-switched pulsed 1064 nm Nd:YAG short pulse laser having a pulse width of 200 ns and a 1552 nm diode short pulsed laser having a pulse width of 1.3 ps. Experimental measurements of axial and radial temperature distribution in the tissue medium are compared with the numerical modeling results. For numerical modeling, the transient radiative transport equation is first solved using a discrete ordinates method for obtaining the intensity distribution and radiative heat flux inside the tissue medium. Then the temperature distribution is obtained by coupling the bio-heat transfer equation with either hyperbolic non-Fourier or parabolic Fourier heat conduction model. The hyperbolic heat conduction equation is solved using MacCormack’s scheme with error terms correction. It is observed that experimentally measured temperature distribution is in good agreement with that predicted by hyperbolic heat conduction model. The experimental measurements demonstrate that converging laser beam focused directly at the subsurface location can produce desired high temperature at that location compared to that produced by collimated laser beam for the same laser parameters. Finally the ablated tissue removal is characterized using histological studies as a function of laser parameters.     

Numerical Simulation of Thermal Damage to Living Biological Tissues Induced by Laser Irradiation Based on a Generalized Dual Phase Lag Model

A generalized dual phase lag (DPL) bioheat model based on the nonequilibrium heat transfer in living biological tissues is applied to investigate thermal damage induced by laser irradiation. Comparisons of the temperature responses and thermal damages between the generalized and classical DPL bioheat model, derived from the constitutive DPL model and Pennes bioheat equation, are carried out in this study. It is shown that the generalized DPL model could predict significantly different temperature and thermal damage from the classical DPL model and Pennes bioheat conduction model. The generalized DPL equation can reduce to the classical Pennes heat conduction equation only when the phase lag times of temperature gradient (τ _T ) and heat flux vector (τ _q ) are both zero. The effects of laser parameters such as laser exposure time, laser irradiance, and coupling factor on the thermal damage are also studied.     

A dimensionless analysis of heat and mass transport in an adsorber with thin fins; uniform pressure approach

A numerical study on heat and mass transfer in an annular adsorbent bed assisted with radial fins for an isobaric adsorption process is performed. A uniform pressure approach is employed to determine the changes of temperature and adsorbate concentration profiles in the adsorbent bed. The governing equations which are heat transfer equation for the adsorbent bed, mass balance equation for the adsorbent particle, and conduction heat transfer equation for the thin fin are non-dimensionalized in order to reduce number of governing parameters. The number of governing parameters is reduced to four as Kutateladze number, thermal diffusivity ratio, dimensionless fin coefficient and dimensionless parameter of Γ which compares mass diffusion in the adsorbent particle to heat transfer through the adsorbent bed. Temperature and adsorbate concentration contours are plotted for different values of defined dimensionless parameters to discuss heat and mass transfer rate in the bed. The average dimensionless temperature and average adsorbate concentration throughout the adsorption process are also presented to compare heat and mass transfer rate of different cases. The values of dimensionless fin coefficient, Γ number and thermal diffusivity ratio are changed from 0.01 to 100, 1 to 10^− 5 and 0.01 to 100, respectively; while the values of Kutateladze number are 1 and 100. The obtained results revealed that heat transfer rate in an adsorbent bed can be enhanced by the fin when the values of thermal diffusivity ratio and fin coefficient are low (i.e., α^? = 0.01, Λ = 0.01). Furthermore, the use of fin in an adsorbent bed with low values of Γ number (i.e. Γ = 10^− 5) does not increase heat transfer rate, significantly.     

Hydrogen production by oxidation of aluminum nanopowder in water under the action of laser pulses

The possibility of hydrogen production by the action of laser pulses (1064 nm, 14 ns, 10 Hz, 0.5–6 J/cm²) on a suspension containing 0.03 wt % aluminum (100 nm) in water is shown. It was found that aluminum nanoparticles absorb the laser energy and heat up. As a result of heating, the continuity of the oxide film is disrupted, and metallic aluminum reacts with water. It is shown that there is a complete transformation of aluminum into products (gibbsite, boehmite, bayerite). The maximum hydrogen yield V_m = 8.4 mL does not depend on the energy density of the laser radiation. The time to reach V_m decreases according to the hyperbolic law with increasing laser energy density, reaching 3.5 min at an energy density of 6 J/cm².     

Thermal model of a dish/Stirling systems

This paper presents a global thermal model of the energy conversion of the 10 kW_el Eurodish dish/Stirling unit erected at the CNRS-PROMES laboratory in Odeillo. Using optical measurements made by DLR, the losses by parabola reflectivity and spillage are calculated. A nodal method is used to calculate the heat losses in the cavity by conduction, convection, reflection and thermal radiation. A thermodynamic analysis of a SOLO Stirling 161 engine is made. The Stirling engine is divided in 32 control-volumes and equations of ideal gas, mass and energy conservation are written for each control-volume. The differential equation system is resolved by an iterative method developed using Matlab^™ programming environment. Temperature, mass, density of working gas, heat transfers and the mechanical power are calculated for one Stirling engine cycle of 40 ms and for a constant direct normal irradiation (DNI). The model gives consistent results correctly fitting with experimental measurements.     

Multiscale Investigation of Femtosecond Laser Pulses Processing Aluminum in Burst Mode

Megahertz is the highest femtosecond laser repetition rate that the state-of-the art technology can achieve. In this article, a single femtosecond laser pulse is burst into multiple femtosecond laser pulses to process aluminum. The temporal gap between two consecutive burst pulses is 2 picoseconds, which is much shorter than the temporal gap between two consecutive pulses at the repetition rate of megahertz. By taking the thermophysical scenarios of femtosecond laser induced of electron thermalization, electron heat conduction, electron–phonon-coupled heat transfer and atomic motion into account, a multiscale framework integrating ab initio quantum mechanical calculation, molecular dynamics and two-temperature model are constructed. The effect of femtosecond laser pulse number on the incubation phenomenon is studied. Comparing with the single pulse-processing aluminum film, the femtosecond laser in burst mode leads to smaller thermal stress, which is favorable to reduce the thermal mechanical damage of the material beneath the laser-irradiated surface. Appreciable differences among the simulation results by using electron thermophysical parameters from ab initio quantum mechanical calculation and those from experimental measurement, empirical estimation and calculation are found, indicating the essentials to precisely model the electron thermal response subject to femtosecond laser excitation.     

Conjugate heat transfer of magnetic mixed convection with radiative and viscous dissipation effects for second-grade viscoelastic fluid past a stretching sheet

A radiative and viscous dissipation effects conjugate heat transfer problem of a second-grade viscoelastic fluid past a stretching sheet has been studied. Governing equations for heat conduction equation of a stretching sheet, and continuity equation, momentum equation and energy equation of a second-grade fluid have been analyzed by a combination of a series expansion method, the similarity transformation and a second-order accurate finite-difference method. These solutions are used to obtain distributions of the local convective heat transfer coefficient and the stretching sheet temperature. The ranges of these dimensionless parameters, the Prandtl number Pr, the elastic number E and the conduction–convection coefficient N_cc are from 0.001 to 10, 0.0001 to 0.01, and 0.5 to 2.0, respectively. A parameter, G, which is used to represent the strength of the buoyancy, is present in the governing equations. A parameter, M_n, which represents the strength of the magnetic filed effect, N_r shows the radiation effect are also present in governing equations. Results indicate that elastic effect in the flow may increase the local heat transfer coefficient and enhance the heat transfer of a stretching sheet. In addition, same as results from Newtonian fluid flow and conduction analysis of a stretching sheet, a better heat transfer has obtained with larger N_cc, G, E, and Pr. It shows that a non-Newtonian flow (E = 0.1, E = 0.01) have a good efficiency to reduce heat for a stretching sheet better than a nearly Newtonian flow (E = 0.001).     

Numerical analysis of conjugate heat transfer in a partially open square cavity with a vertical heat source

Conjugate heat transfer in partially open square cavity with a vertical heat source has been numerically studied. The cavity has an opening on the top with several lengths and three different positions. The other walls of cavity were assumed adiabatic. The heat source was located on the bottom wall of cavity and it has got a width such as Printed Circuit Boards (PCB). Steady state heat transfer by laminar natural convection and conduction is studied numerically by solving two dimensional forms of governing equations with finite difference method. The results were reported for various governing parameters such as Rayleigh number (10³ ≤ Ra ≤ 10⁶), conductivity ratio, opening position, opening length, PCB distance and PCB height. The numerical results were discussed with streamlines, isotherms, Nusselt number and velocity profiles on x- and y-directions. It is found that ventilation position has a significant effect on heat transfer.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Thermal performance of a commercial alkaline water electrolyzer: Experimental study and mathematical modeling

In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1 N m³ H₂/h at an overall power consumption of 4.90 kW h/N m³ H₂ is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50–67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174 kJ °C⁻¹. The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3 W m⁻² °C⁻¹. The model has been implemented using ANSYS^® V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages.     

Conjugate heat transfer of mixed convection for viscoelastic fluid past a horizontal flat-plate fin

A conjugate mixed convection heat transfer problem of a second-grade viscoelastic fluid past a horizontal flat-plate fin has been studied. Governing equations include heat conduction equation of the fin, and continuity equation, momentum equation and energy equation of the fluid, have been analyzed by a combination of a series expansion method, the similarity transformation and a second-order accurate finite difference method. Solutions of a stagnation flow (β = 1.0) at the fin tip and a flat-plate flow (β = 0) on the fin surface were obtained by a generalized Falkner–Skan flow derivation. These solutions have been used to iterate with the heat conduction equation of the fin to obtain distributions of the local convective heat transfer coefficient and the fin temperature. Ranges of dimensionless parameters, the Prandtl number (Pr), the elastic number (E), the free convection parameter (G) and the conduction–convection coefficient (N_cc) are from 0.1 to 100, 0.001 to 0.01, 0 to 1.5 and 0.05 to 2.0, respectively. The elastic effect in the flow could increase the local heat transfer coefficient and enhance the heat transfer of a horizontal flat-plate fin. In addition, same as results from Newtonian fluid flow and conduction analysis of a horizontal flat-plate fin, a better heat transfer has been obtained with a larger N_cc, G and Pr.     

Heat transfer enhancement of high temperature thermal energy storage using metal foams and expanded graphite

Latent heat storage (LHS) can theoretically provide large heat storage density and significantly reduce the storage material volume by using the material’s fusion heat, Δh_m. Phase change materials (PCMs) commonly suffer from low thermal conductivities, being around 0.4 W m⁻¹ K⁻¹ for inorganic salts, which prolong the charging and discharging period. The problem of low thermal conductivity is a major issue that needs to be addressed for high temperature thermal energy storage systems. Since porous materials have high thermal conductivities and high surface areas, they can be used to form composites with PCMs to significantly enhance heat transfer. In this paper, the feasibility of using metal foams and expanded graphite to enhance the heat transfer capability of PCMs in high temperature thermal energy storage systems is investigated. The results show that heat transfer can be significantly enhanced by both metal foams and expanded graphite, thereby reducing the charging and discharging period. Furthermore, the overall performance of metal foams is superior to that of expanded graphite.     

Predictions of buoyancy-induced flow in various across-shape concave enclosures

The present study was conducted to numerically investigate the steady laminar buoyancy-driven and convection heat transfer characteristics within three different across-shape concave enclosures for the Prandtl number of 0.71 and 4, the Grashof number range 10⁴ ≤ Gr ≤ 2 × 10⁵_, and the gap range 0 ≤ H₁/H₂ ≤ 0.25. The steady Navier-Stokes equations, governing the flow under Boussinesq approximation, are solved with the dimensionless stream function-vorticity formulation in terms of curvilinear coordinates using the finite difference method. The results show that the effects of various shapes, the strength of the vortex is relatively bigger in the rectangular-rectangular concave enclosure than in the rectangular-circular concave enclosure at the same Grashof number. Heat transfer from the different across-shape concave enclosures is evaluated, and flow and heat transfer characteristics are discussed.     

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

This paper is the second of a two-part study concerning two-phase flow and heat transfer characteristics of R134a in a micro-channel heat sink incorporated as an evaporator in a refrigeration cycle. Boiling heat transfer coefficients were measured by controlling heat flux (q″ = 15.9 − 93.8 W/cm²) and vapor quality (x_e = 0.26 − 0.87) over a broad range of mass velocity. While prior studies point to either nucleate boiling or annular film evaporation (convective flow boiling) as dominant heat transfer mechanisms in small channels, the present study shows heat transfer is associated with different mechanisms for low, medium and high qualities. Nucleate boiling occurs only at low qualities (x_e < 0.05) corresponding to very low heat fluxes, and high fluxes produce medium quality (0.05 < x_e < 0.55) or high quality (x_e > 0.55) flows dominated by annular film evaporation. Because of the large differences in heat transfer mechanism between the three quality regions, better predictions are possible by dividing the quality range into smaller ranges corresponding to these flow transitions. A new heat transfer coefficient correlation is recommended which shows excellent predictions for both R134a and water.     

Bounds on natural convective heat transfer in a porous layer with fixed heat flux

Using the background field variational method, bounds on natural convective heat transfer in a porous layer heated from below with fixed heat flux are derived from the primitive equations. The enhancement of heat transfer beyond the minimal conduction value (the Nusselt number Nu) is bounded in terms of the non-dimensional forcing scale set by the ‘effective’ Rayleigh number (Rˆ) according to Nu ≤ 0.354Rˆ^1/2 and in terms of the conventional Rayleigh number (Ra) defined by the temperature drop across the layer according to Nu ≤ 0.125Ra. It is presented that fixing the heat flux at the boundaries does not change the linear dependence between Nusselt number and Rayleigh number at high Rayleigh number region.