Bulk Temperature Variation Research Articles

Numerical and Experimental Analysis of the ZFC Heat Release from a YBCO Bulk and Validation of YBCO Thermal Parameters

This article presents results from a simple experimental methodology used to determine the amount of heat transferred from an yttrium barium copper oxide (YBCO) bulk to liquid nitrogen (LN2) and LN2 consumption during the process of zero-field cooling (ZFC). The thermal power can be determined from the YBCO bulk temperature variation, which is difficult to measure with accuracy. In this procedure, the thermal power from the YBCO bulk to LN2 is determined from the measured rate of LN2 evaporation, considering the LN2 latent heat. To reduce the influence of room temperature heating and make the LN2 mass variation depend as much as possible on the heat released from the YBCO bulk, a step transient from room temperature into the LN2 is performed. The precision of results is determined from the rate of LN2 evaporation due to room temperature heating with the bulk already cooled by ZFC. The temperature evolution at the bulk lateral surface where the heat transfer is higher is also measured. The results from experimental measurements are compared with 3D finite element analysis (FEA) numerical results. The obtained evolutions of the temperature and thermal power from the YBCO bulk are used to validate YBCO thermal parameters, such as thermal conductivity and specific heat capacity at constant pressure. The YBCO bulk equivalent heat capacity and thermal resistance are determined by analyzing the equivalent first-order thermal lumped parameter circuit based on the obtained evolutions in time of the YBCO temperature and heat transferred to the LN2. The characteristics of dependence of the YBCO thermal resistance and heat capacity with temperature are obtained by correlating their time evolutions with the bulk average temperature evolution in time. The YBCO-specific heat capacity at constant pressure is then calculated by dividing the obtained bulk heat capacity by the bulk mass. The YBCO thermal conductivity is calculated from the obtained thermal resistance considering an equivalent bulk section and length toward the main direction of heat flux.

Performance analysis of a concentrated direct absorption solar collector (DASC) with nanofluids using computational fluid dynamics and discrete ordinates radiation modelling (CFD-DORM)

This study utilises computational modelling to investigate the application of thermal nanofluids in direct absorption solar collectors (DASC), including the effect of optical and thermal properties on the prediction of thermal energy production. A single-phase, computational fluid dynamics (CFD) model has been used to understand the hydrothermal behaviour of nanofluids operated at high temperature. A non-gray discrete ordinate radiation model (DORM) was coupled to the CFD to capture the dependence of optical parameters on the bulk temperature variation. In order to increase the robustness of the numerical modelling approach, the extinction coefficient of the solar participating media, including the reflective and transmissive behaviour of the semitransparent wall, were taken into consideration. Results show that the exergy efficiency is lower when using fully diffusive conditions at the semitransparent wall as opposed to fully specular conditions. Overall, exergy efficiency decreases with the Nusselt number, Nu, (until Nu=10∼12), although it increases with non-dimensional irradiation. When modelling with temperature-dependent as opposed to temperature-independent properties of the working fluid, the Carnot efficiency was found to vary by 0.67% to 3.30%. Additionally, a correlation between the hydraulic entrance length and Reynolds number, Re, (in the range Re<1,000) has been established and this can be applied for the further hydrothermal analysis. This study serves as a basis for future numerical modelling of any volumetric solar receiver that takes into account the various key system performance indicators of the solar participating media.

A new experimental methodology to assess gear scuffing initiation

ABSTRACT This paper investigates the strong coupling between friction power losses and film thickness that prevent from clearly identifying the mechanism of scuffing through classical test procedures. As the film thickness appears of great influence on the phenomena, a new test method is presented, allowing scuffing initiation study with the film thickness as a key parameter. This new test method allows film thickness variation with minimal friction losses and bulk temperature variations. This procedure has been developed with nitrided steels and a synthetic base oil on a twin disks test rig. Even though asperity contact is considered necessary in literature for scuffing, the test result shows that it can be reached in full film lubrication, potentially through the collapse of the oil film. Different test methods allowing triggering scuffing through different parameters are identified, which shows that a variety of parameters is able to influence scuffing.

Numerical Investigation of Convective Heat Transfer to Supercritical Pressure Hydrogen in a Straight Tube

Hydrogen is adopted as coolant for regenerative cooling nozzle and reactor core in nuclear thermal propulsion (NTP), which is a promising technology for human space exploration in the near future due to its large thrust and high specific impulse. During the cooling process, the hydrogen alters its state from subcritical to supercritical, accompanying with great variations of fluid properties and heat transfer characteristics. This paper is intended to study heat transfer processes of supercritical pressure hydrogen under extremely high heat flux by using numerical approach. To begin with, the models explaining the variation of density, specific heat capacity, viscosity, and thermal conductivity are introduced. Later on, the convective heat transfer to supercritical pressure hydrogen in a straight tube is investigated numerically by employing a computational model, which is simplified from experiments performed by Hendricks et al. During the simulation, the standard k–ε model combining the enhanced wall treatment is used to formulate the turbulent viscosity, and the results validates the approach through successful prediction of wall temperature profile and bulk temperature variation. Besides, the heat transfer deterioration which may occur in the heat transport of supercritical fluids is also observed. According to the results, it is deduced that the flow acceleration to a flat velocity profile in the near wall region due to properties variation of hydrogen contributes to the suppression of turbulence and the heat transfer deterioration, while the “M-shaped” velocity profile is more often correlated to the starting of a recovery phase of turbulence production and heat transfer.

Solid-liquid phase change investigation through a double pipe heat exchanger dealing with time-dependent boundary conditions

The use of phase change materials has been seriously recommended due to their high capacity of energy saving and delivering arising from phase change process. In addition, a constant temperature of melting or freezing provides a desirable condition to transfer a large amount of heat in a low-temperature fluctuation. This work attempts to present a numerical study to simulate the solid-liquid phase change considering a double pipe heat exchanger dealing with time-dependent boundary conditions. For this reason, a time dependent boundary condition of the third kind is applied. In a condition which the amplitude and periodicity of both bulk temperature and heat transfer coefficient (HTC) are varying in the sinusoidal form. The PCM container involves two separate sections to insert two different PCMs of RT28HC and RT35. In addition, in order to make up for the low thermal conductivity of PCMs, a porous medium with high thermal conductivity is located in PCM containers. Numerical model benefits the enthalpy-porosity approach based on the finite volume method, which models the phase change in the fixed grid domain. Moreover, in order to accurate simulation, fluid flow arising from Boussinesq approximation in the liquid phase is also considered using PISO algorithm. According to the results, the arrangement of RT35 in section A and RT28HC in section B accelerates the system response to the boundary oscillation. In addition, increasing the periodicity of the bulk temperature variation increases the amount of phase changing process in both sections while varying the same parameter of HTC does not influence the liquid fraction of PCMs considerably.

Monitoring of Spatiotemporal Patterns in the Oscillatory Chemical Reactions with the Infrared Camera: Experiments and Model Interpretation

An infrared camera was used for the first time to monitor the progress of traveling fronts in oscillatory chemical reactions, taking the Belousov-Zhabotinsky (BZ) reaction as the test system. The experiments involved comparative visual imaging and infrared thermography measurements for the thin-layer of the BZ solution in the Petri dish, including both aqueous and gel media, the latter one hindering the convection. Infrared thermography experiments that supply information on the temperature distribution at the solution surface were compared with the bulk temperature variations of the stirred solution with BZ reaction, measured simultaneously with the oscillatory variations of the Pt electrode potential. The experimentally observed correlation between the ferroin catalyst concentration and the temperature distribution was compared with the results of numerical modeling of these distributions in 2-D reactor space, based on the classical Oregonator. Analogous experiments were performed for the oscillatory oxidation of thiocyanates with hydrogen peroxide, catalyzed with Cu(2+) ions, in search of factors causing the development of traveling fronts, previously reported. The inhomogeneous distribution of the free surface temperature that could contribute to surface instabilities was found. Also, periodical increase and decrease in temperature of solution surface was reported. This was interpreted as periodically predominating cooling of the surface in contact with the surroundings because for the bulk, thermally isolated stirred solution, the temperature monotonically increases. In terms of our nine-variable kinetic model of this system, it was possible to identify the reaction steps responsible for the experimentally observed temperature dynamics and ascribe the appropriate heat effects to them. Our results constitute the first contribution to the thermochemical characteristics of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator.

Modelling bulk water temperature in integrated collector storage systems

A simple low cost Integrated Collector Storage Solar Water Heater (ICS-SWH) was proposed for simulation in Scottish weather conditions. The system takes the form of a rectangular-shaped design incorporating the solar collector and storage tank into a single unit and the simplicity of construction and installation make this type of SWH a popular choice of solar water heater. A 3-month experimental and numerical study on the ICS-SWH was undertaken in order to compare the results with a newly developed macro model. Improvements were incorporated into the macro model by differentiating non-finned and finned collectors. The macro model was able to compare the water bulk temperature variation in different SWH collector material, with or without fins, internal temperature and external weather conditions for a given aspect ratio. Numerical results obtained by this new tool were found to be in close agreement with the experimental data. Practical applications: Direct comparison of measured system performances is relatively straight forward; however, predicting the performance of an ICS-SWH at another location or with a different installation configuration is more challenging. A model able to predict water temperature in ICS-SWH would be a useful tool to allow any user to predict the performance of the installed SWH, depending on the orientation and location. Builders could use such a tool to demonstrate the performance of the ICS-SWH and the benefit of such installation.

Heat removal from oscillating flow in a vertical annular channel

In this study, heat removal from a surface, which is located into the reciprocating flow in a vertical annular liquid column, is investigated experimentally. The experiments are carried out for four different oscillation frequencies and three heat fluxes while the amplitude remains constant for all cases. Instantaneous and time-averaged surface and bulk temperature variations are presented. The cycle-averaged values are considered in the calculation of heat transfer using the experimental measurements. Heat removal from the cold surface due to the oscillating liquid column is determined in terms of Nusselt number. Based on the experimental data, an empirical equation is obtained for the cycle averaged Nusselt number as a function of kinetic Reynolds number.

Analysis of ultrahigh molecular weight polyethylene failure in artificial knee joints: Thermal effect on long-term performance

The mechanism resulting in damage to and failure of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts was investigated on clinically retrieved components. The severity of the subsurface damage increased with the length of time that the component had been implanted. A theoretical analysis was developed to account for the generation of subsurface damage based on a heat transfer model. Friction generates surface heat during articulation of total knee systems. Due to the cooling effect of body fluid on the surface, the rise in temperature on the UHMWPE surface is lower than that below the surface. The peak temperature was estimated to occur on a plane positioned about 1 to 2 mm below the surface. This result was similar to the bulk temperature variation observed during in vivo and in vitro studies by other investigators. Although the difference in temperature on and below the surface is only a few degrees, the thermal effect becomes apparent after a long time and may be explained by the viscoelastic behavior of polymers: the temperature–time equivalence. It is therefore suggested that this thermal effect is another contributory factor to material damage, in addition to high stress and oxidative degradation (in appropriate cases). Therefore, any technological efforts aimed at improving the performance of artificial joint prostheses should minimize the thermal effects at the subsurface of the articular components. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 159–164, 1999

Analysis of ultrahigh molecular weight polyethylene failure in artificial knee joints: thermal effect on long-term performance.

The mechanism resulting in damage to and failure of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts was investigated on clinically retrieved components. The severity of the subsurface damage increased with the length of time that the component had been implanted. A theoretical analysis was developed to account for the generation of subsurface damage based on a heat transfer model. Friction generates surface heat during articulation of total knee systems. Due to the cooling effect of body fluid on the surface, the rise in temperature on the UHMWPE surface is lower than that below the surface. The peak temperature was estimated to occur on a plane positioned about 1 to 2 mm below the surface. This result was similar to the bulk temperature variation observed during in vivo and in vitro studies by other investigators. Although the difference in temperature on and below the surface is only a few degrees, the thermal effect becomes apparent after a long time and may be explained by the viscoelastic behavior of polymers: the temperature-time equivalence. It is therefore suggested that this thermal effect is another contributory factor to material damage, in addition to high stress and oxidative degradation (in appropriate cases). Therefore, any technological efforts aimed at improving the performance of artificial joint prostheses should minimize the thermal effects at the subsurface of the articular components.

Finite element analysis of coupled conduction and convection in refrigerated transport

As an alternative to the continuous analysis of coupled velocity and temperature fields, the average fluid velocities and the convective heat transfer coefficients are estimated first by standard engineering procedures. Afterwards, these values are specified as part of the input data in a finite element code that calculates the detailed temperature distributions in the solid and the bulk temperature variations in the fluid. In the examples, the finite element formulation is validated by comparison with a typical benchmark solution, and the capabilities of the finite element method are demonstrated by the analysis of coupled conduction and convection problems in refrigerated transport.

Instrumentation method and analysis of small temperature fluctuation in incore coolant

The aim of this paper is to review a basic detection method which concerns with temperature fluctuation in upper core region. The measurement of small temperature fluctuation, which may be masked by bulk temperature variation, was carried out by obtaining two small frequency signal components using one thermocouple and a band pass filter systems after eliminating a large signal component. From the signal processing, typical PSD pattern changes were observed according to temperature fluctuation of the coolant. Obtained experimental results are discussed by comparing with theoretical estimation.

Spatial spectra of ocean surface temperature

One-dimensional variance spectra of radiometric ocean surface temperature are similar at small wave numbers to horizontal spectra within the upper ocean. The variance densities in both surface water and upper ocean water decrease continuously with increasing wave numbers. However, at wave numbers greater than 2–5×10−5 cycle cm−1 the spectra within the water continue to decrease, but the surface spectra often become nearly constant. The additional variance density there is attributed to alteration of the surface temperature by patches of compacted organic films. This compaction is produced by convergences in the turbulence within the water. Thus small-scale temperature patterns on the ocean surface often represent only a superficial consequence of the water circulation rather than bulk temperature variations.

The Mean Viscous Dissipation and Bulk Temperature Variation in Incompressible Fully Developed Duct Heat Transfer

The Mean Viscous Dissipation and Bulk Temperature Variation in Incompressible Fully Developed Duct Heat Transfer