An experimental investigation of steam ejector refrigeration system powered by extra low temperature heat source

A steam ejector refrigeration system is a low capital cost solution for utilizing industrial waste heat or solar energy. When the heat source temperature is lower than 80 °C, the utilization of the thermal energy from such a low-temperature heat source can be a considerable challenge. In this investigation, an experimental prototype for the steam ejector refrigeration system was designed and manufactured, which can operate using extra low-temperature heat source below 80 °C. The effects of the operation temperature, the nozzle exit position (NXP) and the diameter of the constant area section on the working performance of the steam ejector were investigated at generating temperatures ranging from 40 °C to 70 °C. Three ejectors with a same de Laval nozzle for the primary nozzle and three different constant-area sections were designed and fabricated. The experimental results show that a steam ejector can function for a certain configuration size of the steam ejector with a generating temperature ranging from 40 °C to 70 °C and an evaporating temperature of 10 °C. For a given NXP, the system COP and cooling capacity of the steam ejector decreased until inoperative as the diameter of the constant area section reduced. The results of this investigation provided a good solution for the refrigeration application of the steam ejector refrigeration system powered by an extra low-temperature heat source.     

A self-adaptive LGSM to recover initial condition or heat source of one-dimensional heat conduction equation by using only minimal boundary thermal data

The present study is concerned with the recovery of an unknown initial condition for a one-dimensional heat conduction equation by using only the usual two boundary conditions of the direct problem for heat equation. The algorithm assumes a function for the unknown initial condition and derives an inverse problem for estimating a spatially-dependent heat source F(x) in T_t(x, t) = T_xx(x, t) + F(x). A self-adaptive Lie-group shooting method, namely a Lie-group adaptive method (LGAM), is developed to find F(x), and then by integrations or by solving a linear system we can extract the information for unknown initial condition. The new method possesses twofold advantages in that no a priori information of unknown functions is required and no extra data are needed. The accuracy and efficiency of present method are confirmed by comparing the estimated results with some exact solutions.     

An optimal strength design of the wound pressure vessel considering nonhomogeneous temperature field

The article presented deals with the optimal strength design of the high pressure compound vessel of mixed construction consisting of the matrix, frequently made of WC–Co, supporting steel ring and outer wound ribbon part. The nonhomogeneous temperature field is taken into account. In the most dangerous working state under the full loading the maximum equivalent stresses at all components equal the corresponding design stresses. The effective stress distribution at the wound part is uniform. A set of equations was derived to calculate all vessel radii as well as the contact pressures.     

Identifying heat conductivity and source functions for a nonlinear convective-diffusive equation by energetic boundary functional methods

Abstract

In the article, we solve the inverse problems to recover unknown space-time dependent functions of heat conductivity and heat source for a nonlinear convective-diffusive equation, without needing of initial temperature, final time temperature, and internal temperature data. After adopting a homogenization technique, a set of spatial boundary functions are derived, which satisfy the homogeneous boundary conditions. The homogeneous boundary functions and zero element constitute a linear space, and then a new energetic functional is derived in the linear space, which preserves the time-dependent energy. The linear systems and iterative algorithms to recover the unknown parameters with energetic boundary functions as the bases are developed, which are convergent fast at each time marching step. The data required for the recovery of unknown functions are parsimonious, including the boundary data of temperatures and heat fluxes and the boundary data of unknown functions to be recovered. The accuracy and robustness of present methods are confirmed by comparing the exact solutions with the identified results, which are obtained under large noisy disturbance.     

Two-dimensional transient conduction and radiation heat transfer with temperature dependent thermal conductivity

This paper deals with the effect of the temperature dependent thermal conductivity on transient conduction and radiation heat transfer in a 2-D rectangular enclosure containing an absorbing, emitting and scattering medium. The thermal conductivity of the medium is assumed to vary linearly with temperature. The radiative part of the energy equation was solved using the collapsed dimension method. To facilitate solution of the energy equation, which is a highly nonlinear one, time linearization was done first and then the equation was solved using the alternating direction implicit scheme. Results for the effects of the variable thermal conductivity were found for temperature and heat flux distributions.     

An average fluid temperature to estimate borehole thermal resistance of ground heat exchanger

Analysis of borehole thermal resistance is important for the standard sizing of ground heat exchanger (GHE). In this paper, a p-linear dimensionless average fluid temperature is proposed to estimate borehole thermal resistance. A p-linear dimensionless fluid temperature and p-linear dimensionless average fluid temperature are introduced, and the p-linear dimensionless fluid temperature is compared with theoretical dimensionless fluid temperature calculated by quasi-three-dimensional model for both single and double U-tubes. Results show that the p-linear dimensionless temperatures with parameters p → 0 and p = ?1/2 are respectively in good agreement with the theoretical dimensionless fluid temperatures of single and double U-tubes. Therefore, the dimensionless logarithmic mean temperature for p → 0 and the dimensionless geometric mean temperature for p = ?1/2 should respectively be adopted to reasonably estimate the thermal resistance of single and double U-tube boreholes.     

Simple measurement of thermal diffusivity and thermal conductivity using inverse solution for one-dimensional heat conduction

A new method is proposed for measuring thermal diffusivity and thermal conductivity simultaneously using the inverse solution for one-dimensional unsteady heat conduction. Unlike previous method proposed by authors, the new procedure does not require the temperature measurement for a long time duration after the temperature starts changing at a sensor position; and then a selection of time duration can be chosen such that the measured temperature change becomes large enough to ensure a required accuracy for the estimated values of thermal diffusivity and thermal conductivity. The measurement is usually completed within 3 min until the temperature rise at the thermocouple position reaches a certain temperature level, for example 1% of an error level. This method has the additional advantage of being independent of the surface condition, except for the requirement of two or three sensing positions in the material. The accuracy of the estimated values is also similar to the error level of the sensor at these positions.     

恒温热源条件下内不可逆布雷顿循环的功率密度特性

用有限时间热力学方法分析恒温热源条件下不可逆布雷顿循环的功率密度特性,计入工质与高、低温侧换热器的热阻损失及压气机、透平的不可逆压缩和膨胀损失,导出了功率密度与压比间的解析式,并通过数值计算将对应于最大功率密度时的一些参数与对应于最大功率时的同样参数进行了比较,说明了功率密度设计的优点与不足.     

Thermal Conductivity Measurement of Semitransparent Media at Temperatures from 300 to 800 K by Hot—Wire Method

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Indirect measurement of surface temperature and heat flux: Optimal design using convexity analysis

A method—convexity analysis—is presented for optimizing the design of indirect measurements involving ill-posed inverse heat conduction problems. Convexity analysis yields a concise, quantitative and physically meaningful assessment of any proposed measurement design. One is thus able to make rational design decisions. Typical questions addressed by convexity analysis in inverse heat conduction problems are: What is the best deployment of a given number of measurements? How does the resolution capability deteriorate as the design is altered from the optimum? What is the utility of the marginal measurement—by how much will the resolution improve if an additional measurement is employed? Quantitative answers to these and other design questions are obtained by implementation of an efficient computerizable min-max algorithm. Design optimization by no means replaces the need for interpretational sophistication, but rather ameliorates the analysis of ill-posed inverse problems.     

An iterative code to investigate heat pump performance improvement by exhaust gases heat recovery

The race between the development of technologies and energy demand has drawn the guidelines of energy strategies for the next two decades. Indeed, the governmental organizations as well as the private sectors are spending huge effort to come up with new adequate strategies that allow to decrease energy consumption. Having said that, heat pump becomes an essential system in our daily life not only in residential building but also in hospital, industrial and touristic building. Nonetheless, (HP)s have very high energy consumption rate. Thus, and to be in line with the new trends in energy strategies, it is convenient to find new methods to enhance the performance of heat pump in order to reduce energy consumption. In this frame, the present paper suggests an approach to enhance the performance the heat pumps using the heat recovery from generators. For this purpose, an in-house code is developed allowing to simulate two new proposed systems (condenser upstream exhaust gases heat recovery system (CU-EGHRS) and condenser downstream exhaust gases heat recovery system (CD-EGHRS). It has been shown that the increase in the performance of the heat pump depends on the capacity of the generator. Also, the CD-EGHRS is shown to be the best. For instance, in the case of a 15 kVA generator, the enhancement could reach 42% for the CD-EGHRS. This enhancement increases to 5640% in the case of a 180 kVA generator.     

Intermediate temperature heat release in an HCCI engine fueled by ethanol/n-heptane mixtures: An experimental and modeling study

This study examines intermediate temperature heat release (ITHR) in homogeneous charge compression ignition (HCCI) engines using blends of ethanol and n-heptane. Experiments were performed over the range of 0–50% n-heptane liquid volume fractions, at equivalence ratios 0.4 and 0.5, and intake pressures from 1.4 bar to 2.2 bar. ITHR was induced in the mixtures containing predominantly ethanol through the addition of small amounts of n-heptane. After a critical threshold, additional n-heptane content yielded low temperature heat release (LTHR). A method for quantifying the amount of heat released during ITHR was developed by examining the second derivative of heat release, and this method was then used to identify trends in the engine data. The combustion process inside the engine was modeled using a single-zone HCCI model, and good qualitative agreement of pre-ignition pressure rise and heat release rate was found between experimental and modeling results using a detailed n-heptane/ethanol chemical kinetic model. The simulation results were used to identify the dominant reaction pathways contributing to ITHR, as well as to verify the chemical basis behind the quantification of the amount of ITHR in the experimental analysis. The dominant reaction pathways contributing to ITHR were found to be H-atom abstraction from n-heptane by OH and the addition of fuel radicals to O₂.     

An artificial intelligence approach to inverse heat transfer modeling of an irradiative dryer

In this work, a new solution approach was developed for heat estimation class of inverse heat transfer problems where radiation provides the dominant mode thermal energy transport. An Artificial Neural Network (ANN) was designed, trained and employed to estimate the heat emitted to irradiative batch drying process.     

Temporal and spatial distribution of heat flux in oscillating flow subjected to an axial temperature gradient

The temporal and spatial distribution of heat flux within counter-oscillating slugs of fluid, along which is maintained a constant axial temperature gradient, is examined. It is found that the resultant axial heat flux pulsates at twice the base oscillation frequency and that the time-averaged axial heat flow under tuned conditions is orders of magnitude larger than that present in the absence of oscillations. Such thermal pumping is produced by the time-dependent interaction of a transverse conduction flux, produced by large transverse temperature gradients, with the periodic axial fluid motion.     

Transient heat conduction in an infinite medium subjected to multiple cylindrical heat sources: An application to shallow geothermal systems

In this paper, we introduce analytical solutions for transient heat conduction in an infinite solid mass subjected to a varying single or multiple cylindrical heat sources. The solutions are formulated for two types of boundary conditions: a time-dependent Neumann boundary condition, and a time-dependent Dirichlet boundary condition. We solve the initial and boundary value problem for a single heat source using the modified Bessel function, for the spatial domain, and the fast Fourier transform, for the temporal domain. For multiple heat sources, we apply directly the superposition principle for the Neumann boundary condition, but for the Dirichlet boundary condition, we conduct an analytical coupling, which allows for the exact thermal interaction between all involved heat sources. The heat sources can exhibit different time-dependent signals, and can have any distribution in space. The solutions are verified against the analytical solution given by Carslaw and Jaeger for a constant Neumann boundary condition, and the finite element solution for both types of boundary conditions. Compared to these two solutions, the proposed solutions are exact at all radial distances, highly elegant, robust and easy to implement.     

基于圆管层流对流传热模型的温度场推算三元非共沸混合物组分方法的研究

给出了一种利用测定圆管对流传热温度场求解确定成分的非共沸混合物的组分的方法。基于定压力条件下液体低雷诺数圆管层流充分发展段流体力学特性,利用数值方法求解变热参数对流传热模型温度场特性,采用反问题方法对对流传热控制方程的热参数进行非线性模拟和反算,通过反演计算热参数得到物质组分的定量值。对非线性热参数条件的对流传热控制方程和反问题L-M方法进行了误差分析,通过试验比较,测定了对流传热过程温度场的理论计算组分与预设的组分。结果表明,测定温度场推算的非共沸混合物组分与实际测定结果满足预设误差。     

An experimental study on convection heat transfer from an array of discrete heat sources

Convection heat transfer from an array of discrete heat sources inside a rectangular channel has been investigated experimentally for air. The lower surface of the channel was equipped with 8×4 flush-mounted heat sources subjected to uniform heat flux; the sidewalls and the upper wall were insulated and adiabatic. The experimental parametric study was made for an aspect ratio of AR=2, Reynolds numbers 864≤Re_Dh≤7955, and modified Grashof numbers Gr*=1.72×10⁸ to 2.76×10⁹. From the experimental measurements, surface temperature distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these temperatures were investigated. Furthermore, Nusselt number distributions were calculated for different Reynolds and Grashof numbers. Results show that surface temperatures increase with increasing Grashof number and decrease with increasing Reynolds number. However, with the increase in the buoyancy affected secondary flow and the onset of instability, temperatures level off and even drop as a result of heat transfer enhancement. This outcome can also be observed from the variation of the row-averaged Nusselt number showing an increase towards the exit.