Precise dental ablation using ultra-short-pulsed 1552 nm laser

A desktop diode pulsed laser having pulse width of 1.3 ps and wavelength of 1552 nm is utilized for precise targeted ablation of dentin, enamel, and composite material while minimizing thermal damage to the surrounding healthy tissue and nerve endings. A thermal imaging camera is used to measure the dental surface temperature rise during ablation. Following ablation, scanning electron microscopy (SEM) and optical microscopy are used to determine the quality of ablation and the volumetric ablation rate as a function of laser parameters. Surface temperature measurements are compared with the numerical modeling results obtained using the transient heat conduction equation. A good agreement between experimental and modeling results for the surface temperature is obtained which ensures accurate prediction of the temperature distribution throughout the tooth using numerical models. The SEM generates images of precise ablation of each dental material when the optimal laser parameters are used and the sample is scanned at a velocity to limit the number of overlapping pulses. During the ablation process there is minimal collateral damage to the surrounding healthy tissue and minimal heat spread throughout the tooth thus preserving the integrity of the pulp.     

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.     

The measurement of heat transfer from hot surfaces to non-wetting droplets

An experimental method to measure the heat transfer between a hot surface and a non-wetting droplet is reported in this paper. By means of transient, high resolution, infrared microscopy, surface temperature measurements with spatial and temporal resolutions of ～100 μm and ～4 ms, respectively, are obtained, by observing a thin metallic layer from beneath through an infrared-transparent substrate. Data from the infrared camera is generated at each time-step in the form of a set of temperatures, at closely-spaced locations on the surface of the infrared transparent plate. Subsequent solution of the transient thermal conduction equation within the substrate permits all thermal quantities (heat flux, energy, etc.) to be determined. As a typical result, the heat transferred by a 1.5 mm droplet is measured to be 0.19 J, with the heat flux peaking at 3.5 MW/m² during the 10 ms it spends in the vicinity of the surface, and with a peak transient surface temperature reduction of 47 °C. Error analysis indicates that the uncertainty in this measurement of heat transfer is about 15%.     

Modeling of heat transfer and energy analysis of potato slices and cylinders during solar drying

In the present work, a method based on energy balance considering the effects of heat capacity of the food product, radiative heat transfer from food product to the drying chamber and solar radiation absorbed in the product during drying is proposed for determination of convective heat transfer coefficient, h_c. A natural convection mixed-mode solar dryer is used for performing the experiments on potato cylinders and slices of same thickness of 0.01 m with respective length and diameter of 0.05 m. The present investigation indicates that the cylindrical samples exhibit higher values of h_c and faster drying rate compared to those of slices, as expected. The h_c values for each sample shape are correlated by an equation of the form Nu = C(Ra)ⁿ. Laplace transform is applied to solve the proposed heat transfer diffusion model considering the effect of moisture transfer rate to predict the transient sample temperature. The model is validated through a close agreement between calculated and experimental results of transient sample temperature. Results of energy analysis reveal that for both the sample geometries, decreasing product moisture content during drying resulted in significant reduction in specific energy consumption. For almost similar drying conditions, a considerable amount of reduction in specific energy consumption is achieved for cylinders, as expected.     

Unsteady conjugate forced convection heat/mass transfer in ensembles of spherical particles with cell models

Numerical methods are used to investigate the transient, conjugate, forced convection heat/mass transfer in multiparticle systems at low to moderate Reynolds numbers. The interparticle interactions have been accounted for by using the simple cell models. The momentum and heat/mass balance equations were solved numerically in spherical coordinates system by a finite difference method. The values considered for the sphere Reynolds number are Re < 100. The computations were focused on the influence of the voidage and physical properties ratios on the heat/mass transfer rate for sphere Peclet number, 10 ? Pe ? 1000.     

Simulation and application of temperature field of carbon fabric wet clutch during engagement based on finite element analysis

The temperature plays a significant role in the tribology properties and failure of friction materials during engagement of wet clutch. In order to obtain the temperature field of carbon fabric wet clutch, the thermal model was developed and the finite element analysis was conducted with the heat flux, convective and conductive heat-transfer taken into account. The predicted temperatures of thermometer hole were compared with experimental values. The effects of the thermal parameters on the temperatures of engagement and the damage of carbon fabric composites were investigated. Results show the thermal is evaluated as effective and can well predict the temperature field. The lower skeletal density, lower specific heat capacity and higher thermal conductivity are indispensable for the purpose of lowering the temperature of engagement. The highest temperature appears at R = 0.0509 m, where the damage of friction lining easily occurs.     

Rewetting and maximum surface heat flux during quenching of hot surface by round water jet impingement

The transient cooling of hot stainless steel surface of 0.25 mm thickness is done with round water jet impingement. Initially, the surface was heated up to the temperature of 800 °C before the water was injected through straight tube type nozzle of 2.5 mm diameter and 250 mm length. During impingement cooling, the surface temperature was measured up to 12 mm radial distance away from the stagnation point. The jet exit to surface spacing, z/d, and jet Reynolds number, Re, varied in the range of 4–16 and 5000–24,000 respectively. The surface rewetting and transient heat flux of the test-surface was studied for these operating parameters.During impingement cooling process the initial rewetting occurred at stagnation region with the lowest wetting delay period. In fact, the rewetting temperature, rewetting velocity and the maximum heat flux reduced for extreme spatial location. However, the wetting delay increased significantly for the locations away from the stagnation point. The surface rewetting and transient heat flux were increased with the rise in jet Reynolds number, resulting in the enhancement in rewetting temperature, rewetting velocity and reduced wetting delay. The maximum heat flux was obtained for 4–6 mm radial location. The effect of jet exit to surface spacing on the rewetting parameters is found to be marginal. A correlation has been developed which predicted the maximum heat flux within an error band of ±10%.     

Numerical study of transient conjugate heat transfer of a turbulent impinging jet

This study presents the numerical study of transient conjugate heat transfer in a high turbulence air jet impinging over a flat circular disk. The numerical simulation of transient, two-dimensional cylindrical coordinate, turbulent flow and heat transfer is adopted to test the accuracy of the theoretical model. The turbulent governing equations are resolved by the control-volume based finite-difference method with a power-low scheme, and the well-known low-Re κ–ω turbulence model to describe the turbulent structure. The SIMPLE algorithm is adopted to solve the pressure–velocity coupling. The parameters studied include turbulent flow Reynolds number (Re = 16,100–29,600), heated temperature of a circular disk (T_h = 373 K) or heat flux (q″ = 63–189 kW/m²), and orifice to heat-source spacing (H/D = 4–10). The numerical results of the transient impinging process indicate that the jet Reynolds number has a significant effect on the hydrodynamics and heat transfer, particularly in the stagnation region of an impinging jet. High turbulence values lead to greater heat transfer coefficients in the stagnation region and cause a bypass of the laminar-to-turbulent transition region in the wall jet region. Induced turbulence from the environment around the jet also influences the variation of the stagnation heat transfer. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.     

Heat transfer enhancement from protruding heat sources using perforated zone between the heat sources

This study is to experimentally investigate the heat transfer enhancement by perforation in air cooling of two in-line rectangular heat sources module. Two separation distances between the heat sources were investigated at s/L = 0.5 and 1.0. The area between the heat sources in both cases were perforated in aligned arrangement such that the holes open area ratio (β) are of 0, 0.0736, 0.1472 and 0.2944. The dimensionless temperature distribution and the average Nusselt number are considered at different values of Reynolds number (3391 ? Re_L ? 10798) and holes open area ratio. It could be seen that perforation could enhance the heat transfer coefficients and reduce the module temperature significantly. Correlations are obtained for the average Nusselt number utilizing the present measurements within the investigated range of the different parameters.     

Predicting the transient response of a serpentine flow-field PEMFC: II: Normal to minimal fuel and AIR

The three-dimensional (3D) transient model presented in part I is used to study the overshoot and undershoot behavior observed in a PEMFC during operation with fixed normal stoichiometic flow rates of hydrogen and air for a 1.0 V s⁻¹ change in the load. In contrast to the behavior with excess flow shown in part I, the predictions show second-order responses for both decreases and increases in the load. That is, there is current overshoot when the load cell is decreased from 0.7 V to 0.5 V and there is current undershoot when the cell voltage is increased from 0.5 V to 0.7 V. The simulation of a 10 cm² reactive area with a serpentine flow path is used to explain this behavior in terms of the reacting gas concentrations, the flow through the gas diffusion media, the movement of water through the MEA by electro-osmotic and back diffusion forces, and the variation in the distributions of current density. The operating conditions correspond to 101 kPa, 70 °C cell temperature, anode and cathode dew-points and stoichiometries of 65 °C and 57 °C and 1.45 and 2.42 at an initial operating voltage of 0.7 V and current density of 0.33 A cm⁻². The fixed flow rates correspond to stoichiometries of 1.05 and 1.73 at 0.5 V for the 0.46 A cm⁻² predicted current density. The predictions illustrate regions where the MEA may alternate between wet and dry conditions and this may be useful to explain stability and durability of the MEA during transient operation.     

The auto-ignition of single n-heptane/iso-octane droplets

Numerical simulations including detailed chemical and physical models are performed to investigate the influence of different physical parameters on the auto-ignition of n-heptane/iso-octane droplets in air. Simulations are performed for isobaric conditions with an ambient pressure of 8 bar and a droplet radius of 200 μm. The ambient gas temperature ranges from 800 K to 2000 K and the droplet temperature was varied from 300 K to 400 K. Below an ambient temperature of 1000 K the ignition delay time is found to increase with an increasing volume fraction of iso-octane. Above 1000 K the ignition delay time appears to be almost independent of the mixture composition of the droplet. The local ignition conditions are also studied. It turns out that ignition occurs at points, where the mixture is lean. This trend is more significant, if the ambient temperature increases. The influence of physical properties of the mixture components, like diffusion coefficients, heat conductivity, heat of vaporization and vapor pressure, is investigated. Furthermore, the influences of simplifying assumptions such as the distillation and diffusion limit are studied.     

Measuring moisture sorption and diffusion kinetics on proton exchange membranes using a gravimetric vapor sorption apparatus

The understanding of water sorption and diffusion properties of proton exchange membranes is crucial to the fuel cell's ultimate performance. In this study, a dynamic gravimetric vapor sorption (DVS) instrument was used to measure the water vapor sorption properties of three Nafion^® based fuel cell membranes: N-117 (extruded film, 183 μm thick); N-112 (extruded film, 51 μm thick); and NR-112 (dispersion cast film, 51 μm thick). Water sorption characteristics were studied between 0 and 95% relative humidity (RH) at 30, 40, 50, 70, and 80 °C. The thicker dispersion cast, N-117, film had a lower water vapor sorption capacity (based on percentage weight gain) than the thinner, N-112 sample. The dispersion cast, NR-112, film had a lower percentage water uptake than the extruded, N-112, film. Below 80% RH, the water sorption capacity increases with temperature for all three samples. Above 80% RH, the moisture sorption capacity increases from 30 to 50 °C, but decreases at 70 and 80 °C compared to the lower temperature data. Moisture diffusion coefficients were also calculated over the humidity and temperature range studied. In general, maximum diffusion coefficients were measured at intermediate humidities. Water heat of sorption calculations at low coverages yielded higher values for the extruded (N-112) film compared to the dispersion cast (NR-112) film indicating a higher affinity for water.     

Time evolution of double-diffusive convection in a vertical cylinder with radial temperature and axial solutal gradients

The characteristics of transient double-diffusive convection in a vertical cylinder are numerically simulated using a finite element method. Initially the fluid in the cavity is at uniform temperature and solute concentration, then constant temperature and solute concentration, which are lower than their initial values, are imposed along the sidewall and bottom wall, respectively. The time evolution of the double-diffusive convection is investigated for specific parameters, which are the Prandtl number, Pr = 7, the Lewis number, Le = 5, the thermal Grashof number, Gr_T = 10⁷, and the aspect ratio, A = 2, of the enclosure. The objective of the work is to identify the effect of the buoyancy ratio (the ratio of solutal Grashof to thermal Grashof numbers: N = Gr_S/Gr_T) on the evolution of the flow field, temperature and solute field in the cavity. It is found that initially the fluid near the bottom wall is squeezed by the cold flow from the sidewall, a crest of the solute field forms and then pushed to the symmetry line. In the case of N > 0, a domain with higher temperature and weak flow (dead region) forms on the bottom wall near the symmetry line, and the area of dead region increases when N varies from 0.5 to 1.5. More crests of the solute field are formed and the flow near the bottom wall fluctuates continuously for N < 0. The frequency of the fluctuation increases when N varies from −0.5 to −1.5. Corresponding to the variety of the thermal and solutal boundary layers, the average rates of heat transfer (Nu) at the sidewall remain almost unchanged while the average rates of mass transfer (Sh) at the bottom wall change much in the cases of N = 1, 0, −1.     

Unsteady heat transfer from an elliptic cylinder

Numerical methods are used to investigate the transient heat transfer from an elliptic cylinder to a steady stream of viscous, incompressible fluid. The temperature of the cylinder is considered spatially uniform but not constant in time. The momentum and heat balance equations were solved numerically in elliptic coordinate system. The solutions span the parameter ranges 5 ? Re ? 40, 1 ? Pr ? 100 and axis ratio ε, 0.1 ? ε ? 0.75. The computations were focused on the influence of the axis ratio and volume heat capacity ratio on the heat transfer rate.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

Transient and isothermal characteristics of a particular heat pipe

This paper presents a simple and rapid mathematical model to calculate the non-steady-state startup process and study the isothermal characteristics of a particular heat pipe. The model takes into consideration the special structure and usage conditions, where vapor temperature in the heat pipe changes only over time. This vapor temperature change correlation is calculated numerically and is set as the temperature boundary condition for the working well. The temperature, velocity and pressure distribution in the working well are then solved using FLUENT. The results manifest that the time required for approaching steady condition are 450 s, 550 s and 600 s with water bath temperatures of 330 K, 340 K and 350 K, respectively. The comparison of the calculations and experimental data shows good agreement, and the maximum deviation is 3.7 K.     

Modelling of an adsorption system driven by engine waste heat for truck cabin A/C. Performance estimation for a standard driving cycle

This paper presents the main characteristics of an innovative cooling system for the air conditioning of a truck cabin, as well as a first estimation of its performance during a standard driving cycle, obtained with a specifically developed vehicle-engine-cooling system overall model. The innovative cooling system consists of a water–zeolite adsorption–desorption system, which employs the waste heat from the engine to produce the cooling of the vehicle cabin. The developed global model is completely dynamic and is able to: reproduce the operation of the engine through a standard driving cycle, evaluate the waste heat available at the engine hydraulic loop; calculate the sequential operation of an adsorption–desorption system, calculate the condensed water per cycle, the cooling effect produced at the evaporator, and finally, the temperature and humidity evolution of the air inside the cabin. The model was validated by experimental data. The experimental tests were performed in a lab-scale adsorption chiller prototype specifically designed and realized to be driven by the low grade waste heat (80–90 °C) from the engine coolant loop of a truck. The experimental activity carried out showed that the chiller is able to generate up to 5 kW of peak cooling power at 10 °C (35 °C of condensation temperature) with a COP of 0.6. The obtained results show that the system could be able to provide a significant amount of the required cooling.     

Effect of synchronized piezoelectric fans on microelectronic cooling performance

This study reports on the influence of dual vibrating fans on flow and thermal fields through numerical analyses and experimental measurements. Two piezoelectric fans were arranged face to face and were vertically oriented to the heat source. 3D simulation was performed with FLUENT and ABAQUS with the use of code coupling interface MpCCI to calculate the velocity and temperature distribution on the horizontal hot plate. The fans' motion was described as deformable parts by ABAQUS at their first mode vibration. The effects of vibration phase difference between the fans corresponding to in-phase (Φ = 0°) and out-of-phase (Φ = 180°) vibrations were explored in terms of transient temperature and flow fields. The purpose is to enhance heat dissipation from the microelectronic component. Comparison with the performance of a single fan is made to assess the significance of the additional fan on thermal performance. Good comparison results were achieved through accurate modeling of the most important features of the fans and through heat transfer. Computed results show that the single fan enhanced heat transfer performance within approximately 2.3 times for the heated surface. By contrast, the dual fans enhanced heat transfer performance within approximately 2.9 for out-of-phase vibration (Φ = 180°) and 3.1 for in-phase vibration (Φ = 0°).     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.