Experiment on the effect of Pt-catalyst on the characteristics of a small heat-regenerative CH4–air premixed combustor

Heat-regenerative small combustors consisting of a combustion space and a pair of counter-current-channels were fabricated. The methane–air combustion characteristics in the combustion space were examined with and without catalytic platinum wires. The installing locations and scale effects of the platinum wires were varied to distinguish between thermo-fluidic and catalytic effects. Temperature distribution through the channels and the compositions of burned gas were measured. Conclusively, the platinum wires in the upstream channel slightly enhanced flame stabilization by an enhanced heat transfer, not by a catalytic reaction. In contrast, the platinum wire located within the recirculation area of the combustion space promoted the catalytic reaction and extended the self-sustainable reaction conditions when it had sufficient surface area. Two reaction regimes, of an ordinary gas reaction and of a catalytic reaction, were distinguished and a hysteresis in the reaction-mode transition was confirmed through the comparison of CO, O₂, and methane across the transition conditions.     

Heat transfer to small cylinders immersed in a packed bed

Fluidized beds have seen increasing use for heat-treating steel wires as an environmentally friendly alternative to molten lead heat-treating systems. System upsets can result in defluidization (packed bed condition) in all or portions of the bed, affecting heat transfer rates to the wires. Existing information regarding heat transfer to cylinders in a packed bed is sparse and contradictory when applied to small cylinder sizes appropriate to wire (1–10 mm). This study examines heat transfer to cylinders immersed in packed beds in size ranges appropriate to steel wire heat-treating applications. An appropriate correlation is developed and presented.     

Heat transfer characteristics of Si and SiC nanofluids during a rapid quenching and nanoparticles deposition effects

Experiments were conducted to investigate the effect of nanofluid on a boiling heat transfer during a rapid quenching of a thin platinum (Pt) wire. The typical overall boiling curves have been successfully obtained from the cooling curves of the Pt wire for the water, the silicon (Si) and the silicon carbide (SiC) nanofluids. Meaningful differences in the behavior of the boiling curves between the water and the nanofluids cannot be identified during a quenching. However, the Si nanofluids reveal a slightly higher CHF (critical heat flux) than that for the water. On the other hand, a slight deterioration of CHF is observed in the case of the SiC nanofluids. When the Si and SiC nanoparticle-coated Pt wires are quenched with water, a very high cooling rate is observed and a very different boiling curve from that of the water and the nanofluids appears. Consequently, a considerably large heat transfer coefficient is obtained in a wide range of the wall superheat in the boiling curve.     

Characteristics of heat transfer coefficients during nucleate pool boiling of binary mixtures

Experimental studies were made on heat transfer on a horizontal platinum wire during nucleate pool boiling in nonazeotropic refrigerant binary mixtures at pressures of 0.25 to 0.7 MPa and at heat fluxes up to CHF. The boiling features of the mixtures and the single-component substances were observed by photography. The relationship between the boiling behavior and the reduction of heat transfer coefficients in binary mixtures is discussed in order to propose a correlation useful for predicting the present experimental data over a wide range of low to high heat fluxes. It is shown that the correlation is applicable to alcoholic mixtures. The physical meaning of k, which was introduced to evaluate the effect of heat flux on the reduction of a heat transfer coefficient, is clarified based on measured nucleate pool boiling heat transfer data and visual observations of the boiling features. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 535–549, 1998     

Natural convection to horizontal wires — The conduction limit

Free convection measurements were carried out from horizontal platinum wires of 0.0254, 0.0508 and 0.0826 cm in diameter, all 10.2 cm in length, to a 2500 wppm aqueous Carbopol 960 solution. For this highly viscous fluid there was no evidence of fluid motion and the measured fluid temperature distribution and heat transfer rates were in good agreement with the values predicted for pure conduction. Similar results were found for highly concentrated (5000 wppm) aqueous polyacrylamide solutions.     

Thermal and water transfer in PEMFCs: Investigating the role of the microporous layer

The objective of this work was to investigate experimentally the effects of the microporous layer (MPL) within a PEMFC. The experiments consisted in measuring, at the anode and at the cathode, the average temperature of the electrodes using small platinum wires, heat fluxes using heat flux sensors and water fluxes by means of water balance for two builds of cell; one with porous layers and MPL and another without MPL. Three thermal configurations related to the imposed temperature of the plates were studied. The measurements put forward a new role of the microporous layer on heat transfer. Indeed, the MPL implies an increase of the electrodes temperature by adding a thermal resistance. This higher temperature enables to avoid the saturation at the electrodes and improve the water removal towards the flow field plates. In addition, the effective thermal conductivity of microporous layer, a key parameter for the analysis of heat transfer in the fuel cell, was estimated in situ.     

A correlation for heat transfer during laminar natural convection in an enclosure containing uniform mixture of air and hydrogen

In this paper, a correlation for heat transfer due to laminar natural convection in a rectangular enclosure containing a uniform mixture of hydrogen and air with vertical walls at different temperatures is proposed. This correlation is in terms of Nusselt and Rayleigh numbers evaluated by taking properties of air alone. Mixture properties, viz., density, viscosity, specific heat capacity and thermal conductivity are not needed in this correlation. A modification factor which is based on the mole fraction of hydrogen in the mixture accounts for the differences in heat exchange due to differences in the properties of mixture and pure air. Thus, this correlation is easier to use for a dilute mixture of hydrogen and air as compared to the conventional correlations that are based on mixture properties which may be cumbersome to evaluate. Further, the results highlight that heat transfer correlations for a mixture of gases can be expressed in terms non-dimensional numbers for dominant gas and a correction factor for the gas mixing with it. An in-house code HDS (Hydrogen Distribution Simulator), which has well validated modules for calculation of mixture properties, has been used to carry out numerical study and establish this heat transfer correlation.     

Thermo-Electrical Mathematical Model for Prediction of Ni-Cr Hot-Wire Temperature in Free Air and Inside Small Circular Cavities

A one-dimensional thermo-electrical mathematical model describing the heating and cooling of thin Ni-Cr20% wires is presented. The model is applied for wires in a free air environment and to wires placed in small circular cavities formed by expanded polystyrene material. The basis of the model is a semicoupled solution of the heat conduction equation and the electrical diffusion equation in a one-dimensional (1-D) control volume finite-difference framework. A study on the available natural convection correlations for thin metal wires for Rayleigh numbers in the range of 10^?3–10¹ is carried out in order to select an appropriate heat transfer coefficient for the time-dependent heating and cooling of a wire. The model is tested against experimental data and is found to be in a good agreement with previous investigations. Based on the findings, expressions for the heat transfer coefficient of a hot wire inside a small circular cavity are suggested.     

超临界压力下航空煤油在竖直管内的传热研究

对竖直上升管内超临界压力下航空煤油的传热特性进行了实验研究。分析了不同质量流量、热流密度、压力和进口温度对超临界压力下航空煤油传热特性的影响。实验结果表明,提高质量流量或进口温度均使煤油传热效果变好。而热流密度对流体传热的影响主要在于改变了流体和壁面温度,热流密度越大,传热系数越高。压力对煤油传热影响不大,一般情况下,提高压力会恶化传热。超临界状态下,煤油物性变化很大,因此对煤油的传输和热力学性质的准确计算是研究超临界压力下传热现象的关键。利用拓展的对比态法来计算煤油的密度和传输特性,如黏度、热导率等。给出了煤油在超临界压力下的传热关联式,其计算值和实验值吻合良好。     

Experimental and modeling analysis of thermal characteristics in carbon fiber wires

To understand the thermal characteristics of carbon fiber heating wires, we tested the heating rules of carbon fiber heating wires with inner core tows of 12k and 24k and different lengths. We found that the surface temperature of the carbon fiber heating wires rose and fell quickly when the power supply was turned on and off by 82.9 and 18.3°C/min, respectively. After the quick rise period, the surface temperature tended to be steady with ±2°C of fluctuation; the steady temperature decreased as the length increased. With an increase in the length from 7 to 12 m, the steady temperature of the 12k and 24k wires reduced from 122°C and 166°C to 53°C and 71°C, respectively. Using the COMSOL Multiphysics software to simulate the carbon fiber heating wires, the simulated surface temperatures were in good agreement with the experimental results, with errors less than 7%. The convective and radiative heat transfer of the simulated carbon fiber electric heating wire decreased with an increase in length and the number of tows, and the convective heat transfer was much higher than the radiative heat transfer to indicate that the convective heat transfer is the main heat transfer mode in the heat dissipation of the carbon fiber electric heating wire. Results indicate that there is a rapid increase in temperature when power is on, and thereafter temperature remains constant. These are the important characteristics of carbon fiber heating wires.     

Pool boiling heat transfer of refrigerant mixtures R32/R125

Nucleate pool boiling heat transfer of new, environmentally harmless refrigerant mixtures R32/R125 has been systematically investigated in a wide range of pressure and heat flux under saturation conditions using a horizontal platinum wire (d = 0.1 mm). The platinum wire served as both heating element and resistance thermometer. The experimental results are compared with calculated values using a modified Jungnickel correlation. The comparison between all calculated data and experimental data shows good agreement. Most of the data fall within ±20% of the correlation. The mean average deviation is ±8.25%.     

Convective heat transfer from small cylinders to mercury

External heat transfer coefficients from short quartz-coated cylindrical hot-film anemometer probes in mercury were evaluated by subtracting the thermal impedance of the quartz coatings from the overall thermal impedance of the sensors. The results are compared with those by Sajben for long enamelcoated wires and with available theory for infinite cylinders, and correlation equations are given. The low sensitivity of finite cylinders may be qualitatively explained by the persistence of end losses, i.e. by two-tauches conduction in the velocity-potential surfaces at arbitrary velocities. For comparison, results are also given for heat transfer to water.     

Experimental Study of Heat Transfer Enhancement for Fluid Flow Inside Annulus with Spiral Wires

The evaluation of heat transfer and pressure drop in a water flow inside an annulus of a double concentric-tube heat exchanger with spiral wires inserts was carried out. Three spiral wires with a constant pitch and different wire diameter were tested for a Reynolds number from 1500 to 5500 and a Prandtl number from 5 to 8. The results obtained showed that the spiral wires increased the heat transfer and the pressure drop in comparison with a fluid flow inside a smooth annulus. From the heat transfer point of view, this increase was proportional to the wire diameter but the effect decreases when the Reynolds number increases. General empirical correlations based on dimensionless parameters to calculate the convective heat transfer coefficient and friction factor were developed with an uncertainty of ±6.1% and ±7.6%, respectively, when these estimates were compared against experimental data. The empirical correlations developed were also compared with the estimates calculated by empirical correlations proposed by other researchers, resulting in a good agreement with these values. After the validation analysis, it was demonstrated that the new equations developed provide a good and reliable tool for the design of double concentric-tube heat exchangers with spiral wires inserted inside annulus.     

Development of Empirical Correlation for Heat Transfer to Nanofluids

Nanofluids are emerging as alternative fluids for heat transfer applications due to enhanced thermal properties. Several correlations are available in open literature for heat transfer coefficient (HTC) and thermophysical properties of nanofluids. Reliability of correlations that use effective properties for estimation of HTC needs to be checked. Comparison of experimental HTC and that estimated from existing correlations is the main objective of the present study. An empirical correlation is developed with experimental data of the HTC for zinc–water and zinc oxide–water nanofluids. Experimental HTC is compared with that estimated from developed correlation and existing correlations. The range of Re considered for the study is 4000 to 18,000. Comparison indicated large deviation in experimental values and the values estimated from existing correlations. Based on comparison results, it can be concluded that the single‐phase models of forced convective heat transfer cannot be extended to nanofluids.     

Enhancement of cooling rate during rapid quenching of a thin wire by ultrasonic vibration

Experiments were conducted to study the effect of ultrasonic vibrations on heat transfer during the rapid quenching of thin horizontal platinum wires in subcooled water and ethanol. The frequency of ultrasonic vibration was 24 and 44 kHz. The power input to the transducer P ranged from 0 to 280 W. The measured boiling curve had two minimum-heat-flux points; the first (named the MI point) corresponded to the onset of significant liquid–solid contact. For P ≤ 20 W, the wall superheat at the MI point increased considerably with increasing P. The heat flux was not much affected by the ultrasonic vibrations until the MI point was reached. After the MI point, the heat flux increased significantly with increasing P. The effect was more significant for υ = 24 kHz. Distributions of the sound pressure and the cavitation intensity were also measured and their effects on the heat transfer characteristics are discussed. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 16–30, 1998     

Influence of thermodynamic models on the prediction of pool boiling heat transfer coefficient of dilute binary mixtures

In this paper, large number of experiments has been performed on saturated pool boiling heat transfer to three different dilute binary mixtures at various heat fluxes (up to 100 kW/m²) and five different concentrations (1–5 vol.% of heavier component). The test mixtures include water/glycerol, water/mono‐ethylene glycol (MEG), and water/di‐ethylene glycol (DEG). The effects of the main operating parameters such as heat flux, concentration, and bubble dynamics on the pool boiling heat transfer coefficient are also investigated. Furthermore, it is shown that physical properties of the mixtures have a considerable effect on the prediction of pool boiling heat transfer coefficients using the available correlations. In almost all of the existing correlations, some physical properties are strongly involved which can be estimated using different thermodynamic models. These models for the calculation of specific heat, density, heat of vaporization, and surface tension do not give exactly similar results and consequently, the heat transfer coefficient obtained from a specified predictive correlation can be tolerated according to the method used for the calculation of the physical properties. This point is usually ignored by investigators and they compare their experimental data with the correlations without reporting which thermodynamic models have to be used for the calculation of the physical properties. In this study, the prediction of Schlünder correlation has been compared with the present experimental data. Results show that the prediction ability of the Schlünder correlation is strongly dependent on the method used for the estimation of the required physical properties.     

The physics of heat transfer from hot wires in the proximity of walls of different materials

This paper concerns the physical process of heat transfer from hot wires located in the proximity of walls consisting of different thermal conductivities. It points out that it is common practice to calibrate hot wires in a free stream of constant and known velocity, but when utilized for near-wall measurements additional heat losses occur owing to the presence of the wall, resulting in erroneous velocity readings. Therefore, a combined experimental and numerical methodology for heat transfer from a heated wire in a flow field is proposed, taking the effects of wire diameter, overheat ratio, wall thermal conductivity, wall distance, wall thickness and shear rate on the measured velocity into account. The present investigations indicated that the flow under the plate, i.e., the corresponding shear rate at the wall opposite the location of the wire where velocity measurements were taken, changes the thermal boundary conditions around the hot wire. It was also observed that heat diffusivity is dominant in the wall region and plays the major role in heat transfer from the wire rather than convection, especially for highly heat-conducting materials. For highly heat-conducting walls, a universal correction law for the wall influence was given by Durst et al. [F. Durst, E.-S. Zanoun, M. Pashtrapanska, In situ calibration of hot wires close to highly heat-conducting walls, Exp. Fluids 31 (2001) 103–110]. However, for poorly heat-conducting walls, the correction law depends on the wall thickness and the heat transfer from the surface opposite the wall where the hot-wire measurements were performed.     

Transient natural convection heat transfer of liquid D-mannitol on a horizontal cylinder

Transient and steady state natural convection heat transfer for D-mannitol on a horizontal cylinder was investigated experimentally at various liquid temperatures and heat input conditions. To clarify the natural convection phenomena of D-mannitol, transient and steady heat transfer coefficients were measured under various liquid temperatures of D-mannitol and periods of heat generation rates from a horizontal platinum cylinder. The platinum cylinder with a diameter of 1 mm and a length of 43.5 mm was used as the test heater in this experiment. Experimental results indicated that the steady heat transfer coefficient of D-mannitol was affected by the liquid temperature. As the liquid temperature increased, it was understood that the effect of liquid temperature weakened. When the period of the heat generation rate was changed, the heat transfer process was divided into natural convection heat transfer and conductive heat transfer. It was considered that the conductive heat transfer was more dominant as the period of the heat generation rate decreased. The empirical correlations of steady and transient heat transfer coefficients for D-mannitol were obtained.     

Heat transfer enhancement and pressure drop of the horizontal concentric tube with twisted wires brush inserts

In the present study, the heat transfer characteristics and the pressure drop of the horizontal concentric tube with twisted wires brush inserts are investigated. The inner diameters of the inner and outer tubes are 15.78 and 25.40 mm, respectively. The twisted wire brushes are fabricated by winding a 0.2 mm diameter of the copper wires over a 2 mm diameter of two twisted iron core-rod with three different twisted wires densities of 100, 200, 300 wires per centimeter. The plain tube with full-length twisted wires brush and regularly spaced twisted wires brush with 30 cm spacer length inserts are tested. Cold and hot water are used as working fluids in shell side and tube-side, respectively. The test runs are performed at the hot water Reynolds number ranging between 6000 and 20000. The inlet cold and hot water temperatures are 15, 20 °C, and between 40 and 50 °C, respectively. Effect of twisted wires density, inlet fluid temperature, and relevant parameters on heat transfer characteristics and pressure drop are considered. Twisted wire brushes insert have a large effect on the enhancement of heat transfer, however, the pressure drops also increase.     

Development of Heat Transfer Correlation for Supercritical Water in Vertical Upward Tubes

Supercritical water is widely used in many advanced single-phase thermosiphons due to its favorable heat and mass transfer characteristics and potentially high thermal efficiency. However, the heat transfer characteristics of supercritical water in the deterioration regime cannot be accurately predicted due to the absence of exact evaluation of the effect on steep variation in thermophysical properties near the pseudocritical point. The present paper focuses on the deterioration mode by analyzing the physical mechanism and constructing a new correlation. About 3,000 experimental data on supercritical water, including 40 deteriorated heat transfer cases from open literature, were collected. Quantitative assessment of heat transfer behavior was conducted based on existing test data and previous criteria gathered from extant literature. Based on experimental data evaluation and phenomenological analysis, an improved dimensionless correlation is proposed by introducing multi-dimensionless parameters, which can correct the deviation of heat transfer from its conventional behavior in the Dittus-Boelter equation. Comparisons of various heat transfer correlations with the selected test data show that the new correlation agrees better with the test data versus other correlations selected from the open literature.