High Temperature Difference Research Articles

Using infrared thermography to investigate thermomagnetic convection under spatial non-uniform magnetic field

Thermomagnetic convection was investigated experimentally by infrared thermography and validated numerically by a finite-element analysis. A cavity filled with a magnetic liquid was under the influence of a permanent magnet that provided an external magnetic field and was subjected to thermal forcing by maintaining the two opposite horizontal walls at different temperatures. All configurations were chosen to explore situations where the Kelvin body force and buoyancy would either oppose each other or act perpendicular to each other. As both body forces depend on temperature, the aim was to explore the degree by which heat transfer may be enhanced by only varying the temperature difference. The results demonstrate that the Kelvin body force, which can be much stronger than buoyancy, leads to convective flow where only conduction would occur without the presence of the magnet. An effective force ratio, r, is developed which suggests that an increase in temperature difference across the cavity initially leads to a dominance of thermomagnetic forcing over buoyancy but that the overall force balance becomes more moderate at higher temperature differences. The global effect of both body forces on heat transfer was characterised by the Nusselt number and a suitable modified Rayleigh number.

Microstructure, mechanical properties and thermal shock resistance of ZrB 2 -SiC-C f composite with inhibited degradation of carbon fibers

Abstract ZrB 2 -SiC-C f composite with carbon coated carbon fibers was successfully prepared by low temperature hot pressing. This composite showed an excellent thermal shock resistance compared with those composites fabricated by hot pressing and spark plasma sintering using as-received carbon fibers. Toughening mechanisms such as crack deflection, crack branching, fibers pull-out and fibers bridging were obviously detected in ZrB 2 -SiC-C f owing to the inhibited degradation of carbon fibers and the relatively weak fiber/matrix interfacial bonding, leading to a non-brittle fracture mode of the composite. Moreover, the composite exhibited an excellent thermal shock resistance with a high critical thermal shock temperature difference of 773 °C which is about twice those of the reported ZrB 2 -based ultra-high temperature ceramics. This work provides valuable guidance in the preparation of ZrB 2 -based ultra-high temperature ceramics with a combination of excellent properties.

Comparative study of discrete velocity method and high-order lattice Boltzmann method for simulation of rarefied flows

Abstract A comparative study of discrete velocity method (DVM) and high-order lattice Boltzmann method (HLBM) is made in this work. The key difference between the DVM and HLBM lies in the use of equilibrium distribution function and discrete velocity. In the DVM, the complete Maxwellian distribution function or the Shakhov distribution function is used and the discrete velocity is to represent the continuous velocity domain in the particle velocity space. As a result, the integral forms for the moments of distribution function are approximated by numerical integration. In order to make the quadrature error to be small enough, a large number of discrete velocity points are required for the DVM in general case. In contrast, in the HLBM, the lattice Boltzmann model is utilized as the equilibrium distribution function and discrete velocity. The construction of lattice Boltzmann model is intended to seek a minimal set of velocities in the particle velocity space to represent the objective hydrodynamics. With lattice properties of the lattice Boltzmann model, the weighted sums of distribution functions at those velocity points can exactly satisfy the physical conservation laws. However, the choice of discrete velocity points may have considerable effect on the accuracy of HLBM for fluid flows with large Knudsen number as reported in literatures [26, 29–30, 34–36]. From this study, it was found that HLBM provides reasonable results for fluid flows at moderate Knudsen number and low temperature difference with less computational cost than that of DVM. For fluid flows with large Knudsen number and/or high temperature difference, the DVM is preferred although more computational cost is required.

Effect of cyclic temperature impact on coal seam permeability

The gas permeability of the coal samples, subjected to cyclic temperature impact under different conditions, was considered in the article. Meanwhile, the change of the coal sample permeability and fractures were analyzed through scanning electron microscopy. The obtained results show that the cyclic temperature impact has largely positive effect on the increase of the coal sample permeability and that new fractures and permeability growth increase with the higher temperature difference.

A design study of high buffer pressure low temperature difference type Stirling engine

A design study of high buffer pressure low temperature difference type Stirling engine

Stability of thermoviscous fluid flow under high temperature gradients

In present study the influence of temperature-dependent viscosity, i.e. thermoviscosity on the pressure-driven flow in inhomogeneous temperature field has been recalled. Such viscous stratification impacts on the velocity profile asymmetry, significantly increases the entry length, and causes the velocity inflection point to appear. In certain cases, particularly under high temperature differences of channel walls, the thermoviscosity generates instability that in a view of high local Reynolds numbers can create turbulent zones.

Exergy Analysis of a Novel Inlet Air Cooling System with gas Turbine Engines using cascaded Waste-heat Recovery

In this work, an innovative sustainable energy efficient system is dealt with cascaded waste-heat recovery system (CWHRS). It is composed of a gas turbine (GT) engine, where the heat was recovered from exhaust gases in an upper propane ORC cycle which produces power from the first expander. This heat is further recovered in another lower propane gas refrigeration cycle, where the most important target is cooling the ambient intake air of the GT engine. Thus, engine power is boosted and thermal efficiency is improved. An exergy analysis is carried out, where the primary objective is to analyse each component and overall system, hence determining where the major losses of exergy occur. Many operating variables are taken into consideration: namely, inlet temperature of expander 1, turbine inlet temperature, pressure ratio for the GT, upper and lower propane cycles. The analysis shows that at design point the overall exergy efficiency and destruction are 37% and 95,952 kW, respectively. Moreover, the highest amount of irreversibility occurs in the combustion chamber due to the high temperature difference in the chemical reaction. The optimisation analysis reveals best exergy efficiency with about 40%.

Thermoelectric generator experimental performance testing for wireless sensor network application in smart buildings

In order to make a conventional building more efficient or smarter, systems feedbacks are essential. Such feedbacks can include real-time or logged data from various systems, such as temperature, humidity, lighting and CO2 levels. This is only possible by the use of a network of sensors which report to the building management system. Conventional sensors are limited due to wiring and infrastructure requirements. Wireless Sensor Networks (WSN) however, eliminates the wiring limitations but still in certain cases require periodical battery changes and maintenance. A suitable solution for WSN limitations is to use different types of ambient energy harvesters to power battery-less sensors or alternatively to charge existing batteries so as to reduce their changing requirements. Such systems are already in place using various energy harvesting techniques. Thermoelectric Generators (TEG) are one of them where the temperature gradient is used to generate electricity which is conditioned and used for WSN powering applications. Researchers in this field often face difficulty in estimating the TEG output at the low-temperature difference as manufacturers’ datasheets and performance data are not following the same standards and in most cases cover the high-temperature difference (more than 200C°). This is sufficient for industrial applications but not for WSN systems in the built environment where the temperature difference is much smaller (1-20C° is covered in this study). This paper presents a TEG experimental test setup using a temperature controlled hotplate in order to provide accurate TEG performance data at the low-temperature difference range.

Performance comparison between the combined R134a/CO2 heat pump and cascade R134a/CO2 heat pump for space heating

In this paper, the performance of combined R134a/CO2 and cascade R134a/CO2 systems for space heating was studied and compared under specific operating conditions. A mathematical model was developed for this purpose and was verified by experimental data. The effects of key parameters such as water feed temperature, water supply temperature and ambient temperature on the performance of the two systems were further investigated. Results showed that cascade and combined systems had different preferable application conditions. The cascade system performed better at low ambient temperatures while the combined system performed better under conditions of high ambient temperature and high hot water temperature differences between the system inlet and outlet. A correlation was established and an operating condition coefficient was proposed to determine which system should be used for any given set of working conditions.

Thermography and machine learning techniques for tomato freshness prediction.

The United States and China are the world's leading tomato producers. Tomatoes account for over $2 billion annually in farm sales in the U.S. Tomatoes also rank as the world's 8th most valuable agricultural product, valued at $58 billion dollars annually, and quality is highly prized. Nondestructive technologies, such as optical inspection and near-infrared spectrum analysis, have been developed to estimate tomato freshness (also known as grades in USDA parlance). However, determining the freshness of tomatoes is still an open problem. This research (1) illustrates the principle of theory on why thermography might be able to reveal the internal state of the tomatoes and (2) investigates the application of machine learning techniques-artificial neural networks (ANNs) and support vector machines (SVMs)-in combination with transient step heating, and thermography for freshness prediction, which refers to how soon the tomatoes will decay. Infrared images were captured at a sampling frequency of 1 Hz during 40 s of heating followed by 160 s of cooling. The temperatures of the acquired images were plotted. Regions with higher temperature differences between fresh and less fresh (rotten within three days) tomatoes of approximately uniform size and shape were used as the input nodes for ANN and SVM models. The ANN model built using heating and cooling data was relatively optimal. The overall regression coefficient was 0.99. These results suggest that a combination of infrared thermal imaging and ANN modeling methods can be used to predict tomato freshness with higher accuracy than SVM models.

Progress in and prospects for electrical insulating materials

The performance of electrical equipment and devices is determined to a great extent by the properties of their insulating materials. In power systems and electrical devices, insulating materials have to work in extreme circumstances that can include high temperature differences, intense radiation, and strong electric fields. Such conditions demand high-quality insulating materials with superior electrical, thermal, and mechanical properties as well as resilience to other environmental stresses. This study focuses on advances in insulating materials since the early 20th century and reviews the many developments in their properties and applications, including electric breakdown strength, thermal conductivity, temperature resistance, corona resistance, and specific energy storage in dielectrics. Some research progress on other properties is also covered, such as non-linearity and radiation resistance. Investigations into the properties of insulating materials can greatly assist in understanding their interface effects and composite structures, which in turn is helpful for discovering methods to improve the performance of electrical devices. Future directions for research are proposed to guide new investigations and support the development of even better insulating materials.

Implications of atmospheric conditions for analysis of surface temperature variability derived from landscape-scale thermography

Thermal infrared (TIR) cameras perfectly bridge the gap between (i) on-site measurements of land surface temperature (LST) providing high temporal resolution at the cost of low spatial coverage and (ii) remotely sensed data from satellites that provide high spatial coverage at relatively low spatio-temporal resolution. While LST data from satellite (LSTsat) and airborne platforms are routinely corrected for atmospheric effects, such corrections are barely applied for LST from ground-based TIR imagery (using TIR cameras; LSTcam). We show the consequences of neglecting atmospheric effects on LSTcam of different vegetated surfaces at landscape scale. We compare LST measured from different platforms, focusing on the comparison of LST data from on-site radiometry (LSTosr) and LSTcam using a commercially available TIR camera in the region of Bozen/Bolzano (Italy). Given a digital elevation model and measured vertical air temperature profiles, we developed a multiple linear regression model to correct LSTcam data for atmospheric influences. We could show the distinct effect of atmospheric conditions and related radiative processes along the measurement path on LSTcam, proving the necessity to correct LSTcam data on landscape scale, despite their relatively low measurement distances compared to remotely sensed data. Corrected LSTcam data revealed the dampening effect of the atmosphere, especially at high temperature differences between the atmosphere and the vegetated surface. Not correcting for these effects leads to erroneous LST estimates, in particular to an underestimation of the heterogeneity in LST, both in time and space. In the most pronounced case, we found a temperature range extension of almost 10 K.

Novel Concept of Fluidized Bed Reactor Design for Highly Efficient Internal Heat Transfer

A novel double pipe fluidized bed (DPFB) reactor was used in the present study to supply heat energy during fast pyrolysis. The steady state heat transfer model of the DPFB was developed where classical equations of heat transfer inside the fluidized bed and that of wall-to-bed were considered. The sensitivity analysis showed that contact time was a critical parameter to the overall heat transfer coefficient. The model provided good predicting accuracy on temperature calculations by deviating from measured data within +20%. The simulation of DPFB was conducted based on the model and results showed that the heat exchange mainly occurred in the bed phase. Both reducing contact time and increasing Reynolds number would improve heat transfer significantly. The effect of static bed height on outlet temperatures of DPFB could be observed under high inlet temperature difference, which illustrated the importance of the annular fluidized bed in heat transfer enhancement.

Two-dimensional modeling of a plant-scale fixed-bed reactor for hydrogen production from methanol steam reforming

Methanol as hydrogen carrier can be reformed with steam over CuO/ZnO/Al2O3 catalysts. Two-dimensional pseudo-homogenous modeling and numerical simulation studies have been carried out for methanol steam reforming reactor integrating with global kinetics of methanol decomposition and methanol steam reforming. Hydrogen output and thermal behavior were investigated with respect to parameters such as mass flow rate and inlet mass fraction of methanol, external thermal oil inlet temperature and tube diameter. Good agreement was achieved between plant data and simulated data. The results show that temperature distribution changed more dramatically than concentration distribution along axial. Hydrogen production had a positive relationship with methanol mass flow rate, thermal oil inlet temperature. Thermal behavior along radial can be greatly affected by all discussed operating conditions. Large tube diameter was adverse to heat transfer leading to high temperature difference. These simulations can guide for manufacture operation and design of plant-scale reactor.

Design, fabrication and feasibility analysis of a thermo-electric wearable helmet

This paper presents a wearable thermoelectric generation system which is applied to a helmet. It involves the use of the relatively high temperature difference between the local effect of the head skin and the ambient to generate electricity for powering the LEDs. Meanwhile, the mathematical model of the thermoelectric generator (TEG) and the heat transfer process of the thermo-electric system are established and analyzed. The properties of the head underneath the helmet are also investigated and analyzed. Furthermore, the commercially available modules have been tested and the performance of energy conversion of it has been evaluated experimentally. And the models are verified via the experiments. Models of TEG matched with LED have been certified by the experiment results are presented to optimize the number of the TEG for maximum output and the related circuits are designed. The thermo-electric wearable helmet is manufactured. In addition, the performance of the system under different environmental conditions are evaluated and predicted. The results indicate the feasibility of the wearable thermoelectric generation system and give hints for future improvement of the wearable devices using outside under different environmental conditions.

Performance of Thermoelectric Module as a Water Cooler and Water Heater

This paper presents the total Coefficient of Performance (CoP) of a thermoelectric module in combined heating and cooling modes, obtained through experiments and thermodynamics mathematical model. Thermoelectric module is a solid state heat pump which has a capability to pump heat with the capacity of 100 W or lower. One of the module surface will acts as a heat sink while another surface of the module rejects heat. This provides an opportunity to utilize both surfaces for cooling and heating applications. The objective of this work was to determine the total CoP of the thermoelectric module when it was operated in a combination of heating and cooling modes. The total CoP was inferred by fitting the temperature of cold and hot water streams which were obtained from experiments to a mathematical model of the thermoelectric heating and cooling system. The highest temperature difference between hot and cold streams was 40 Kelvin. The worst root-mean-square error between the measured and calculated temperature was 2 Kelvin, except for the hot stream temperature curve in the Condition 1(RMS error was 3.5 Kelvin). It was proved that the total CoP of 4.5 was achievable for thermoelectric module operated in the combined heating and cooling modes. However, the total CoP decreased as the temperature difference between hot and cold streams was increased. It can be concluded that the use of thermoelectric module in heating and cooling modes can improve its CoP which contribute to saving in electrical consumption by the module.

Numerical investigation of laminar Rayleigh-Bénard convection of Bingham fluids in square cross-sectioned cylindrical enclosures

Numerical simulations have been conducted under the assumption of axisymmetry to analyse steady-state laminar natural convection of yield stress fluids obeying Bingham model in square cross-sectioned cylindrical annular enclosures heated from below (i.e. Rayleigh-Bénard configuration). The simulations have been carried out for a representative value of nominal Prandtl number (i.e. Pr = 500) for different internal cylinder radius (0≤ri/L≤16 where ri and L are the inner radius and the cylinder height respectively) for a nominal Rayleigh number range 103≤Ra≤105. Both constant wall temperature and constant wall heat flux boundary conditions have been imposed for differentially heated horizontal walls to analyse the effects of wall boundary condition. Although the buoyancy-driven transport strengthens with increasing Ra, the mean Nusselt number Nu¯cy does not show a monotonic increase with increasing Ra for small values of ri/L because of the change in flow pattern (i.e. number of convection rolls/cells). By contrast, Nu¯cy monotonically increases with increasing Ra, and only one cell flow pattern is obtained for large values of ri/L. Furthermore, Nu¯cy has been found to increase with increasing ri/L but asymptotically approaches the corresponding value obtained for square enclosures (ri→∞) for both CWT and CWHF boundary conditions for large values of ri/L. It has also been found that both the flow pattern and the mean Nusselt number Nu¯cy are dependent on the initial conditions for Bingham fluid cases since hysteresis is evident for small values of ri/L for both CWT and CWHF boundary conditions. It has been found that convection could be sustained up to a higher value of Bingham number due to stronger convection arising from higher temperature difference between horizontal walls in the case of CWT boundary condition than in the corresponding CWHF configuration. Finally, the numerical findings have been used to propose a correlation for Nu¯cy in the range of 2≤ri/L≤16 (0.25≤ri/L≤16) and 103≤Ra≤105 for the CWT (CWHF) boundary condition.

Particle size and type effects on heat transfer enhancement of Ferro-nanofluids in a pulsating heat pipe

The present study evaluated the effect of γ (gamma) and α (alpha) Fe2O3/Kerosene nanofluids for a closed loop pulsating heat pipe under the magnetic field. The nanoparticles had a size ranged from 10nm to 30nm. The heat transfer rate and the temperature distribution of the heat pipe were examined with and without the magnetic field. Likewise, the Fe2O3 base nanofluids were exposed to a magnetic field to measure the vapor temperature at the center of the pulsating heat pipe directly. The results showed that both heat transfer coefficient and thermal performance of the pulsating heat pipe are enhanced by the addition of Fe2O3 nanoparticles, especially when the magnetic field is present. The increased input heat flux rises the heat transfer coefficient of the condenser and the evaporator. Lower evaporator heat transfer coefficient and higher temperature difference between condenser and evaporator were seen because of α-Fe2O3 nanoparticles as compared to γ-Fe2O3 nanoparticles. Among six nanoparticles investigated in this research, the optimum type and size of Iron oxide nanoparticles under similar conditions for attaining the best heat transfer performance was 20nm γ-Fe2O3 nanoparticles.

Performance and cold spot effect of methanol steam reforming for hydrogen production in micro-reactor

Catalytic hydrogen production by methanol steam reforming is considered to be an attractive hydrogen source option. However, performance of conventional packed beds for methanol steam reforming may be limited by heat transfer, which results in a low effective factor of the catalyst. In this work, CuO/ZnO/Al2O3 catalyst particles with diameter of 0.5 mm were filled in the micro-reactor for hydrogen production by methanol steam reforming. At steam to methanol ratio of 1.3, effects of inlet temperature, space velocity on reactor performance were investigated. The temperature distribution in the catalyst bed has also been studied. Results showed that methanol conversion decreased with space velocity and it increased with inlet temperature. Hydrogen production yield increased firstly and then decreased with space velocity. And it increased with the increasing of temperature. However, CO content in the product also increased with temperature, and it decreased with the increasing of inlet space velocity, especially at low temperature condition. Compared to conventional tubular fixed bed reactor, the micro-reactor presented a slightly better performance due to the fixed bed nature of catalyst, which is inefficiency in tubular reactor. Cold spot was observed at inlet of micro-reactor reaction chamber, and the highest temperature difference at cold spot area reached about 6 K. The maximum temperature difference throughout the catalyst bed reached 10 K in the micro-reactor.

Mathematical Modelling of Stress-Strain State of Multilayer Shell Molds

The analysis of the trends for development of technological processes to form shell molds shows that the accuracy and purity of the surface of shaped castings are one of the main criteria for the level of development of the science and technology of foundry engineering. The equations of continuum mechanics and thermal conductivity and the numerical method were used to build a mathematical model of SSS of multilayer shell mold to determine the temperature fields in a shell mold and solidifying casting, thickness of solidified layer of the casting, stress, strain, and displacement of shell layers against each other, as well as ferrostatic pressure on the metal form. The calculation results show that at metal casting, the highest temperature differences occur between the first and second layers of a shell mold, and the maximum values of the stresses and strains are typical for the first layer of a shell mold. The mechanism of the effect of temperature difference between the layers of the shell mold on fracture toughness of the shells and its critical value are determined, above which either a decomposition of a shell mold occur, or through-thickness cracking occur, which lead to the destruction of a shell mold. The ignition process of shell molds in the control filler has no significant influence on the formation of cracks, since the temperature difference between the shell layers does not reach critical values.