Plate fin-tube heat exchangers fins are bonded with tubes by means of brazing or by mechanical expansion of tubes. Various errors made in the process of expansion can result in formation of an air gap between tube and fin. A number of numerical simulations was carried out for symmetric section of plate fin-tube heat exchanger to study the influence of air gap on heat transfer in forced convection conditions. Different locations of air gap spanning 1/2 circumference of the tube were considered, relatively to air flow direction. Inlet velocities were a variable parameter in the simulations (1– 5 m/s). Velocity and temperature fields for cases with air gap were compared with cases without it (ideal thermal contact). For the case of gap in the back of the tube (in recirculation zone) the lowest reduction (relatively to the case without gap) of heat transfer rate was obtained (average of 11%). The worst performance was obtained for the gap in the front (reduction relatively to full thermal contact in the average of 16%).
The paper describes experimental research on a resistojet type rocket thruster which was built as an actuator in the Attitude Control System of a model space robotic platform. A key element of the thruster is the heater responsible for increasing the temperature of the working medium in the thruster chamber and hence the specific impulse. This parameter describes the performance of the thruster, increases providing – for lower propellant consumption – the same propulsion effect (thrust). A high performance thruster means either total launch mass can be reduced or satellite lifetime increased, which are key commercial factors. During the first phase of the project, 7 different heating chamber designs were examined. The heater is made of resistive wire with resistivity of 9Ω/m. Power is delivered by a dedicated supply system based on supercapacitors with output voltage regulated in the range of 20–70 V. The experimental phase was followed by designing the chamber geometry and the heating element able to deliver both: maximum increase of gas temperature and minimum construction dimensions. Experiments with the optimal design show an increase in temperature of the working gas (air) by about 300 ◦C giving a 40% increase in specific impulse. The final effect of that is a 40% reduction in mass flow rate while retaining thrust at a nominal level of 1 N.
This study discusses results of experiments on hydrodynamic assessment of gas flow through backbone (skeletal) porous materials with an anisotropic structure. The research was conducted upon materials of diversified petrographic characteristics – cokes. The study was conducted for a variety of hydrodynamic conditions, using air. The basis for assessing hydrodynamics of gas flow through porous material was a gas stream that results from the pressure forcing such flow. The results of measurements indicate a clear impact of the type of material on the gas permeability, and additionally – as a result of their anisotropic internal structure – to a significant effect of the flow direction on the value of gas stream. In aspect of scale transfer problem, a method of mapping the flow geometry of skeletal materials has been developed and usefulness of numerical methods has been evaluated to determine pressure drop and velocity distribution of gas flow. The results indicate the compliance of the used calculation method with the result of experiments.
The coupled fluid/solid heat transfer computations are performed to predict the temperatures reached in the rotating disc systems. An efficient finite element analysis (FEA) and computational fluid dynamics (CFD) thermal coupling technique is developed and demonstrated. The thermal coupling is achieved by an iterative procedure between FEA and CFD calculations. In the coupling procedure, FEA simulation is treated as unsteady for a given transient cycle. To speed up the thermal coupling, steady CFD calculations are employed, considering that fluid flow time scales are much shorter than those for the solid heat conduction and therefore the influence of unsteadiness in fluid regions is negligible. To facilitate the thermal coupling, the procedure is designed to allow a set of CFD models to be defined at key time points/intervals in the transient cycle and to be invoked during the coupling process at specified time points. The computational procedure is applied to predict heat transfer characteristics of a free rotating disc.
Nowadays, the energy cost is very high and this problem is carried out to seek techniques for improvement of the aerothermal and thermal (heat flow) systems performances in different technical applications. The transient and steady-state techniques with liquid crystals for the surface temperature and heat transfer coefficient or Nusselt number distribution measurements have been developed. The flow pattern produced by transverse vortex generators (ribs) and other fluid obstacles (e.g. turbine blades) was visualized using liquid crystals (Liquid Crystal Thermography) in combination with the true-colour image processing as well as planar beam of double-impulse laser tailored by a cylindrical lens and oil particles (particle image velocimetry or laser anemometry). Experiments using both research tools were performed at Gdańsk University of Technology, Faculty of Mechanical Engineering. Present work provides selected results obtained during this research.
The paper presents a description of used methods and exemplary mathematical models which are classified into theoretical-empirical models of thermal processes. Such models encompass equations resulting from the laws of physics and additional empirical functions describing processes for which analytical models are complex and difficult to develop. The principle of developing, advantages and disadvantages of presented models as well as quality prediction assessment were presented. Mathematical models of a steam boiler, a steam turbine as well as a heat recovery steam generator were described. Exemplary calculation results were presented and compared with measurements.
The production of thermal energy from solar energy by flat collectors finds nowadays many applications due to their innumerable economic and environmental interests. Currently, conservation of energy resources has become a global priority. On the other hand, given the dizzying demand for energy, has led specialists to find new techniques, such as renewable energies (solar, wind and geothermal). The present work is a contribution, by numerical simulation, to the study of heat transfer in flat solar collectors. On the basis of some experimental data, several simulation calculations have been carried out in order to determine the influencing parameters allowing better performance of the sensors and ensuring a good homogeneity of the temperature distributions. Based on the observation that, due to the low thermophysical properties of the air used as heat transfer fluid, solar air collectors rather give poor yields. It has been found very useful to have ‘baffling’ obstacles of various shapes and forms in the solar collector duct. This increases the thermal transfer of a coolant, which clearly improves the thermal efficiency of the solar air collector. This article consists mainly of studying the effects on heat transfer of turbulent forced convection by baffles of zigzag shapes, placed in a rectangular channel, using the finite volume method. The pressure-velocity coupling has been processed by the SIMPLEC algorithm. The results are presented in terms of the average Nusselt number and temperature field for different positions.
The thermodynamic properties, which are the important bulk properties for solids, have been investigated for ZrB2 under pressure through the quasi harmonic Debye model. The dependences of thermal expansion, Gruneisen parameter, Debye temperature and specific heat on pressure P are successfully obtained. The obtained results are in a good agreement with the available experimental and other theoretical data.
Experimental investigation of natural convection heat transfer in heated vertical tubes dissipating heat from the internal surface is presented. The test section is electrically heated and constant wall heat flux is maintained both circumferentially and axially. Four different test sections are taken having 45 mm internal diameter and 3.8 mm thickness. The length of the test sections are 450 mm, 550 mm, 700 mm and 850 mm. Ratios of length to diameter of the test sections are taken as 10, 12.22, 15.56, and 18.89. Wall heat fluxes are maintained at 250–3341 W/m2. Experiments are also conducted on channels with internal rings of rectangular section placed at various distances. Thickness of the rings are taken as 4 mm, 6 mm, and 8 mm. The step size of the rings varies from 75 mm to 283.3 mm. The nondimensional ring spacing, expressed as the ratios of step size to diameter, are taken from 1.67 to 6.29 and the non-dimensional ring thickness, expressed as the ratios of ring thickness to diameter are taken from 0.089 to 0.178. The ratios of ring spacing to its thickness are taken as 9.375 to 70.82. The effects of various parameters such as length to diameter ratio, wall heat flux, ring thickness and ring spacing on local steady-state heat transfer behavior are observed. From the experimental data a correlation is developed for average Nusselt number and modified Rayleigh number. Another correlation is also developed for modified Rayleigh number and modified Reynolds number. These correlations can predict the data accurately within ±10% error.
Heat and mass transfer stretched flow of an incompressible, electrically conducting Jeffrey fluid has been studied numerically. Nanoparticles are suspended in the base fluid and it has many applications such as cooling of engines, thermal absorption systems, lubricants fuel cell, nanodrug delivery system and so on. Temperature dependent variable thermal conductivity with Rosseland approximation is taken into account and suction effect is employed in the boundary conditions. The governing partial differential equations are first transformed into set of ordinary differential equations using selected similarity transformations, which are then solved numerically using Runge-Kutta-Felhberg fourth-fifth order method along with shooting technique. The flow, heat and mass transfer characteristics with local Nusselt number for various physical parameters are presented graphically and a detailed discussion regarding the effect of flow parameters on velocity and temperature profiles are provided. It is found that, increase of variable thermal conductivity, radiation, Brownian motion and thermophoresis parameter increases the rate of heat transfer. Local Nusselt number has been computed for various parameters and it is observed that, in the presence of variable thermal conductivity and Rosseland approximation, heat transfer characteristics are higher as compared to the constant thermal conductivity and linear thermal radiation.
Proposed is the analysis of steam condensation in the presence of inert gases in a power plant condenser. The presence of inert, noncondensable gases in a condenser is highly undesirable due to its negative effect on the efficiency of the entire cycle. In general, thermodynamics has not provided an explicit criterion for assessing the irreversible heat transfer process. The method presented here enables to evaluate precisely processes occurring in power plant condensers. This real process is of particular interest as it involves a number of thermal layers through which heat transfer is observed. The analysis was performed using a simple, known in the literature and well verified Berman’s model of steam condensation in the presence of non-condensable gases. Adapted to the geometry of the condenser, the model enables, for instance, to recognise places where non-condensable gases are concentrated. By describing with sufficient precision thermodynamic processes taking place in the vicinity of the heat transfer area segment, it is possible to determine the distributions of thermodynamic parameters on the boundaries between successive layers. The obtained results allow for the recognition of processes which contribute in varying degrees to irreversible energy degradation during steam condensation in various parts of the examined device.
The purpose of this paper is to depict the effect of diffusion and internal heat source on a two-temperature magneto-thermoelastic medium. The effect of magnetic field on two-temperature thermoelastic medium within the three-phase-lag model and Green-Naghdi theory without energy dissipation i discussed. The analytical method used to obtain the formula of the physical quantities is the normal mode analysis. Numerical results for the field quantities given in the physical domain are illustrated on the graphs. Comparisons are made with results of the two models with and without diffusion as well as the internal heat source and in the absence and presence of a magnetic field.
The work deals with the heat analysis of generalized Burgers nanofluid over a stretching sheet. The Rosseland approximation is used to model the non-linear thermal radiation and incorporated non-uniform heat source/sink effect. The governing equations reduced to a set of nonlinear ordinary differential equations under considering the suitable similarity transformations. The obtained ordinary differential equations equations are solved numerically by Runge-Kutta-Fehlberg order method. The effect of important parameters on velocity, temperature and concentration distributions are analyzed and discussed through the graphs. It reveals that temperature increases with the increase of radiation and heat source/sink parameter.
In this study a cooling ejector cycle coupled to a compression heat pump is analyzed for simultaneous cooling and heating applications. In this work, the influence of the thermodynamic parameters and fluid nature on the performances of the hybrid system is studied. The results obtained show that this system presents interesting performances. The comparison of the system performances with hydrofluorocarbons (HFC) and natural fluids is made. The theoretical results show that the a low temperature refrigerant R32 gives the best performance.
The present work focuses on analyses of the autoignition delay time predicted by the large eddy simulation (LES) method by applying different subgrid scales (SGS) models and two different discretization schemes. The analysed flow configuration is a two-phase chemically reacting turbulent flow with monodispersed evaporating fuel droplets. The impact of numerical procedure is investigated in a 3D flow domain with a temporally evolving mixing layer that constituted between the streams of fuel and oxidizer that moved in opposite directions. The upper stream of cold gas carries a dispersed fuel spray (ethanol at 300 K). The lower stream is a hot air at 1000 K. Three commonly used in LES, SGS models are investigated, namely: classical Smagorinsky model, model proposed by Vreman and the #27;-model proposed by Nicoud. Additionally, the impact of two discretization schemes, i.e., total variation diminishing (TVD) and weighted essentially nonoscillatory (WENO) is analysed. The analysis shows that SGS model and discretization scheme can play a crucial role in the predictions of the autoignition time. It is observed that for TVD scheme the impact of SGS model is rather small. On the contrary, when the WENO scheme is applied the results are much more dependent on the SGS model.
The paper presents the results of the design analysis and experimental investigations of the microturbine set consisting of the microturbine with partial admission and permanent magnet generator. The microturbine was designed for operation with the vapour of ethanol as a working fluid. Microturbine unit was tested for different parameters of the working fluid and varying the electrical load. The examples and the comparison between experiment results and numerical simulations are shown and discussed in the paper.
The paper describes issues related to pressure drop that accompanies the phenomenon of maldistribution of working fluid between the channels of a model minichannel plate heat exchanger. The research concerns a single exchanger’s plate containing 51 (in every geometry) parallel rectangular minichannels of hydraulic diameters 461 μm, 571 μm, 750 μm, and 823 μm. In addition, more complex geometry has been investigated, equipped with additional diagonal channels (so called extended geometry). The moment of the liquid phase transition through the heat exchanger was recorded at the flow rates ranging from 0.83 g/s to 13.33 g/s in the inlet manifold. The paper discusses the total pressure drop as a function of the flow rate and the characteristic dimension of minichannels, as well as the pressure drop as a function of the time of the fluid passage through the main part of the measuring section in which measurements were done. The resulting profiles correlate with the images of the flow distribution between channels recorded using the fast shutter speed camera, that allows to draw a further conclusions about the specifics of the maldistribution process. The impact of the total pressure drop on the actual range of optimum operating conditions of the heat exchanger was analyzed.
This paper reports the results of research involving observations of flow patterns during air-oil-water three-phase flow through a vertical pipe with an internal diameter of 0.03 m and a length of 3 m. The conductometric method based on the measurement of electrical conductivity of the gas-liquid-liquid system was used to evaluate the flow patterns. In the studies, a set of eight probes spaced concentrically in two tube sections (four probes per each) with a spacing of 0.015 m were used. The paper presents a theoretical description of the test method and the analysis of the measurement results for air-oil-water multiphase flow system. Results of this study indicate that the developed method of characterizing the voltage of the gas-liquid-liquid system can be an important tool supporting other methods to identify flow patterns, including visual observation.