Conduction based calibration of handmade platinum thin film heat transfer gauges for transient measurements

Heat transfer measurement using thin film gauges (TFG) is the most prevalently used technique for determination of surface heat flux. They are best suited for short duration transient surface temperature measurements and typically used in the applications where convection is a dominant mode of heat transfer such as gas turbine engines, high speed flights etc. However, in few interdisciplinary research areas, there are practical issues and difficulties in exposing the gauges for convection based measurements. These present investigations are aimed at exploring the possibility of using thin film gauges for short duration conduction based transient measurements with pure conduction mode of heat transfer. A simple calibration set-up has been used to supply known heat flux of different magnitudes to the thin film gauges that are fabricated in-house with platinum as sensing element and pyrex as an insulating substrate. Experimentally recorded temperature signals from the gauges are compared with simulated temperature histories obtained through finite element analysis. Convoluted integral of one-dimensional heat conduction equation is used to predict the surface heat flux and compared with input heat loads. The presently developed calibration setup is seen to be very useful for conduction based measurements of thin film gauges.     

The theory of advanced multi-layer thin film heat transfer gauges

This paper describes the theory of a transient method of measuring heat transfer rate to metal substrates coated with an electrical insulator, using thin film resistance thermometers. This builds on the already well-established system which uses semi-infinite insulating substrates. It is intended that the new technique will have application in rotating turbine test rigs, since there is at present a lack of suitable instrumentation which can be easily manufactured, and which does not interfere with the flow. The new system described here shows that multi-layer substrate gauges can be used. This paper presents analyses of layered gauges and gives sample predictions and calibrations.     

Transient heat flux measurement analysis from coaxial thermocouples at convective based step heat load

The measurement of surface heating rate is an imperative parameter in the force convection ground-based facility for short duration investigation due to the heat transfer rate is changing rapidly. The coaxial thermocouples are suitable to measure the transient heat flux in fast varying heat transfer application because of having fast response time in the range microseconds or less. In this addition, the K-type, E-type, and J-type of coaxial thermocouples are contrived as well as the thermal coefficient resistance (TCR) and sensitivity (S) has been calculated from oil-bath based technique. These handmade coaxial thermocouples are exposed in a forced convection flow facility having three different input step heat loads as well as their transient heat fluxes are estimated using one-dimensional heat conduction modeling for the semi-infinite body. The numerical simulation has also been carried out with the analogous experimental parameters using ANSYS-FLUENT v.15.0 and compared with the outcome of the experimental approach and it is found that the average value of the transient temperatures having 0.3% error and surface heat flux recovered from this temperature is 10%. This study reveals the measuring ability of these handmade coaxial thermocouples at low temperature and low velocity on short duration transient measurements.     

Conjugate heat transfer in inclined open shallow cavities

Conjugate heat transfer in inclined open shallow cavities has been numerically studied. A thick wall facing the opening is heated by a constant heat flux, sides perpendicular to the heated wall are insulated and the opening is in contact with a fluid at constant temperature and pressure. Conjugate heat transfer by conduction and natural convection is studied by numerically solving equations of mass, momentum and energy. The governing parameters were: Rayleigh numbers, Ra from 10⁶ to 10¹², conductivity ratio, k_r from 1 to 60, cavity aspect ratio, A=H/L from 1 to 0.125, dimensionless wall thickness, ?/L from 0.05 to 0.20 and the inclination angle, ? from 0° to 45° from the horizontal. Isotherms and streamlines are produced, heat and mass transfer is calculated. It is found that volume flow rate, is an increasing function of Ra, A, ?/L, ?, and a decreasing function of k_r. Heat transfer, Nu is an increasing function of Ra, ?/L, and a decreasing function of k_r. A mixed pattern is found with respect to A and ?. In the former, Nu is an increasing function of the aspect ratio up to a critical Rayleigh number, above which the relationship changes and it becomes a decreasing function of A. In the latter case, Nu is a decreasing function at low Raleigh numbers and an increasing one at high Rayleigh numbers.     

Natural convection heat transfer in partially open inclined square cavities

A numerical study has been carried out on inclined partially open square cavities, which are formed by adiabatic walls and a partial opening. The surface of the wall inside the cavity facing the partial opening is isothermal. Steady-state heat transfer by laminar natural convection in a two dimensional partially open cavity is studied by numerically solving equations of mass, momentum and energy. Streamlines and isotherms are produced, heat and mass transfer is calculated. A parametric study is carried out using following parameters: Rayleigh number from 10³ to 10⁶, dimensionless aperture size from 0.25 to 0.75, aperture position at high, center and low, and inclination of the opening from 0° (facing upward) to 120° (facing 30° downward). It is found that the volume flow rate and Nusselt number are an increasing function of Rayleigh number, aperture size and generally aperture position. Other parameters being constant, Nusselt number is a non-linear function of the inclination angle. Depending on the application, heat transfer can be maximized or minimized by selecting appropriate parameters, namely aperture size, aperture position and inclination angle at a given operation Rayleigh number.     

Transient analysis of closed loop solar water heating system (effect of heat exchangers)

This paper presents a general and more realistic model of the transient behaviour of a forced circulation solar water heating system with and without heat exchangers in the collector loop and storage tank. The analysis has been presented for a constant flow mode. The effects of heat exchanger length and various water heating system parameters on its performance have been studied. Numerical calculations have been made for a typical cold day (26 January 1980) at Delhi.     

Heat transfer mechanisms in thin film with laser heat source

The present study deals with the effect of laser radiation on the propagation phenomenon of a thermal wave in a very thin film subjected to a symmetrical heating on both sides. Pulsating laser heating is modelled as an internal heat source with various time characteristics. The Cattaneo heat flux law together with the energy conservation equation is solved by a numerical technique based on explicit scheme, i.e., MacCormack’s predictor–corrector scheme. Results are obtained for the time history of heat transfer behaviour before and after symmetrical collision of wave fronts from two sides of a film. The study concludes (1) if the absorption coefficient of the continuously-operated- and pulsating-laser heat source increases, temperature overshoot causes in a very thin film within a very short period of time, and (2) the overshoot and oscillation of thermal wave depend on the frequency of the heat source time characteristics. This trend becomes minor in a thick film.     

Thermal efficiency of evaporative heat loss for open and closed cycle systems

The authors present an analytical expression for the thermal efficiency of evaporative heat loss for open and closed cycle systems in terms of system design and climatic parameters. The derived analytical expression can be used for optimum design of evaporative cooling (open cycle) and distillation system (closed cycle). The theoretical results have also been validated experimentally for open cycle system.     

Conjugate heat transfer in open cavities with a discrete heater at its optimized position

In this article, we determined optimum position of a discrete heater by maximizing the conductance and then studied heat transfer and volume flow rate with the discrete heater at its optimum position in open cavities. Continuity, Navier–Stokes and energy equations are solved by finite difference-control volume numerical method. The relevant governing parameters were: the Rayleigh numbers from 10⁶ to 10¹², the Prandtl number, Pr = 0.7, the cavity aspect ratio, A = H/L from 0.5 to 2, the wall thickness l/L from 0.05 to 0.15, the heater size h/L from 0.15 to 0.6, and the conductivity ratio k_r from 1 to 50. We found that the global conductance is an increasing function of the Rayleigh number, the conductivity ratio, and a decreasing function of the wall thickness. Best thermal performance is obtained by positioning the discrete heater at off center and slightly closer to the bottom. The Nusselt number and the volume flow rate in and out the open cavity are an increasing function of the Rayleigh number and the wall thickness, and a decreasing function of the conductivity ratio. The Nusselt number is a decreasing function of the cavity aspect ratio and the volume flow rate is an increasing function of it.     

Transient surface heating rates from a nickel film sensor using inverse analysis

Convective surface heat transfer measurements play an important role in many industrial, environmental and aerodynamic problems. In most of the cases, the flow is unsteady which results in temperature variation in the body. The surface heating rates are then predicted from the measured temperature histories by suitable heat transfer modeling. In this paper, the temperature history obtained from a nickel film sensor during a flight test is considered to study the effect of sensor thickness on surface heat flux measurements during the flight measurement. Inverse methods using analytical solutions as well as control volume approximations are used to infer the surface heat flux. The experimental temperature data are discretized using cubic-spline method to obtain the closed form solution which is used for inverse analysis. The results are compared with that of standard bench mark results with thin film gauge analysis based on semi-infinite one dimensional medium. No significant change in surface heat flux is observed between inverse and thin film analysis. However, when the thickness of nickel film is increased by 100 times during numerical simulation of inverse method, it is seen that peak surface heat flux increases by 20%.     

Experimental results and a user-friendly model of heat transfer from a thin film resistance temperature detector

The research reported in this paper has focused on the different modes of heat transfer – conductive (to the substrate), conductive and convective (to the environment) and radiative (to the environment) – from an on-chip resistance temperature detector (RTD). The study has been carried out at various input voltages, various pressures ranging from atmospheric to vacuum, and for two classes of platforms for the device – thermal insulators (glass wool and ceramic), and a thermal conductor (aluminum block). The transient temperature–time response of the RTD under the various conditions stated above was recorded. A heat transfer model approximately accounting for all the modes of heat transfer was introduced. The calibration parameters of the model allowed us to quantify the different modes of heat transfer. The model uncovers the fact that the heat losses to the environment via conduction and convection are almost as much as the heat lost by radiation (radiative effects were unequivocally confirmed experimentally). Compared to these losses, conductive heat losses from the RTD to its underlying substructure are far more dominant (almost five times). We also give an analysis originating from the exact form of conservation of energy and demonstrate that the use of the simplified model has led to the most dominant heat transfer mode of conduction to the substrate being underestimated by no more than 7.89% (at the highest input power tested).     

Identification of boundary heat fluxes in a falling film experiment using high resolution temperature measurements

In this paper, we consider a three-dimensional inverse heat conduction problem (IHCP) in a falling film experiment. The wavy film is heated electrically by a thin constantan foil and the temperature on the back side of this foil is measured by high resolution infrared (IR) thermography. The transient heat flux at the inaccessible film side of the foil is determined from the IR data and the electrical heating power. The IHCP is formulated as a mathematical optimization problem, which is solved with the conjugate gradient method. In each step of the iterative process two direct transient heat conduction problems must be solved. We apply a one step θ-method and piecewise linear finite elements on a tetrahedral grid for the time and space discretization, respectively. The resulting large sparse system of equations is solved with a preconditioned Krylov subspace method. We give results of simulated experiments, which illustrate the performance and tuning of the solution method, and finally present the estimation results from temperature measurements obtained during falling film experiments.     

Transient heat conduction in cool-thermal discharge systems with producing chilled air under constant discharge fluxes and external recycle

The mathematical model of the heat transfer phenomena in the cool-thermal discharge systems with external recycle and constant discharge flux has been derived theoretically in this study. The chilled air was produced by the ambient air flowing through the ice layer surface and water layer surface after the ice melting. The conduction–convection heat-transfer problem with a moving boundary was solved using the energy balance associated with integral boundary-layer analysis. The effects of recycle operation on the outlet chilled air temperature, air mass velocity and average Nusselt numbers for three numerical cases with three different discharge fluxes were discussed. The results show that a considerable device performance improvement was obtained by employing external recycle in a cool-thermal discharge system.     

Cooling enhancement in an air-cooled finned heat exchanger by thin water film evaporation

A theoretical analysis on the cooling enhancement by applying evaporative cooling to an air-cooled finned heat exchanger is presented in this work. A two-dimensional model on the heat and mass transfer in a finned channel is developed adopting a porous medium approach. Based on this model, the characteristics of the heat and mass transfer are investigated in a plate-fin heat exchanger with the interstitial surface fully covered by thin water film. Assuming that the Lewis number is unity and the water vapor saturation curve is linear, exact solutions to the energy and vapor concentration equations are obtained. The cooling effect with application of evaporative cooling was found to be improved considerably compared with that in the sensible cooler. This is because the thermal conductance between the fin and the air increases due to the latent heat transfer caused by the water evaporation from the fin surface. It is also found that the cooling enhancement depends greatly on the fin thickness. If the fin is not sufficiently thick, the cooling enhancement by the evaporative cooling decreases since the fin efficiency drops considerably due to the water evaporation from the fin surface. The fin thickness in the evaporative cooler should be increased larger than that in the sensible cooler to take full advantage of the cooling enhancement by the water evaporation.     

Heatline analysis of heat recovery and thermal transport in materials confined within triangular cavities

Heat recovery from hot fluids in material processing industries is important for environmental and thermal management. Present work involves numerical visualization of heat flow in entrapped cavities filled with hot materials. The concept of heatline is used to visualize the heat energy trajectory. The system involves entrapped triangular cavities filled with hot fluid (Pr = 0.015, 0.026, 0.7 and 1000). At low Rayleigh number (Ra), it is found that the heatlines are smooth and perfectly normal to the isotherms indicating the dominance of conduction for both the triangles. As Ra increases, flow slowly becomes convection dominant. Multiple heat flow circulations with high intensity are formed within the lower triangular domain especially for low Pr numbers, whereas, less intense convective heat flow circulations are observed for the upper triangle. Multiple circulations are absent for both the triangular domains involving fluids with higher Pr numbers. It is observed that the heat transfer rates are monotonic for the upper triangle whereas a few local maxima in heat transfer rates occur for smaller Pr within lower triangular domain. Overall, fluid with any Pr may be useful for enhanced heat transfer within the upper triangle but fluid with high Pr may be preferred for the lower triangle.     

Experimental techniques for determining heat and mass transfer due to condensation of humid air in cooled,open cavities

An experimental investigation was conducted to examine combined buoyancy driven heat and mass transfer in open cavities of different aspect ratios. Test cavities were constructed with calorimeter plates bonded to Styrofoam insulation. The inside of the cavities was cooled and then exposed to ambient air for approximately 17–30 min. Measurements were conducted at three initial cavity temperatures (5 °C, 1 °C, and −5 °C) each for a range of ambient relative humidity from 60% to 75%. The mass of retained condensate accumulated on the inside cavity walls and the transient cavity temperatures were measured. The mass flux of retained condensate inside the cavity was compared to the mass flux determined utilizing the heat/mass transfer analogy. The results between the two methods were in good agreement. The effects of the retained condensate on the mass flux measurements due to the water’s thermal resistance and surface emittance were also investigated and shown to have a negligible impact.     

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

This paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ).DOI 10.1002/htj.10050     

Predicting radiative heat fluxes and flammability envelopes from unintended releases of hydrogen

The prediction of the radiative heat flux from a turbulent-jet flame issuing from a damaged, high-pressure hydrogen storage system is an issue of importance for the safe use of hydrogen. Information about the variation of the thermal radiation exposure with distance from the hydrogen jet flame as well as the length and duration of the flame is important in determining safe distances for the handling and storage of hydrogen. An equally important issue is the determination of the concentration decay of an unignited hydrogen jet in the surrounding air, and the envelope of locations where the concentration falls below the lower flammability limit for hydrogen.     

In situ modification by graphidyne as interlayer in titanium dioxide thin film / platinum for water splitting photocatalysis

Modifying titanium dioxide (TiO₂) with graphdiyne (Gry) makes that the absorption range of UV-vis-nIR spectrum expands in visible light area. However, in traditional methods, the two materials are combined by hydrothermal process. In this paper, in-situ synthesis, which monomer directly polymerizes on the surface of TiO₂ instead that polymer and TiO₂ combined by hydrothermal method, is employed to get hybrid materials. In the process, TiO₂ thin film was deposited on the glass slide followed by being enfolded in copper (Cu) foil to initiate polymerization and synthesize Gry on the surface of TiO₂ directly. It turns out that with co-catalyst Platinum (Pt) the efficiency of TiO₂-Gry with introduction of only 1.9 wt% Gry is 12.6% higher than the efficiency of TiO₂ under same test conditions. The improvement is attributed to band structure of the combined material and efficiency of charge transfer. In this system, Gry acts as an excellent medium for charge transfer, which benefits from its intrinsic property and effective contact with TiO₂ by this synthesis. The work points out a new way to get well-combined platelike photocatalyst on transparent substrate. It gives a promising prospect for practical photocatalysis in the future that the quantity and mass ratio of combined materials can be recorded and modified.