Special Heat Exchanger Research Articles

The thermal behaviour of a special heat exchanger filled with the phase change material dedicated for low – temperature storage applications

The work deals with an experimental and theoretical analysis of a special heat exchanger filled with the PCM (Phase Change Material) mixture of paraffin wax capsuled in the melamine-formaldehyde membrane and water, dedicated to low-temperature storage applications. In the first part of the article, the PCM topic was introduced. The next part consists information about the designed and manufactured special heat exchanger with the PCM. During the experiment as a PCM is utilized mixture of paraffin wax in microcapsules within the water. The experimental results of temperature distributions and stored energy in the heat exchanger were compared with the theoretical model proposed in the literature. In the discussion section, the authors describe a new idea of the PCM storage tank application, which could be applied e.g. in nuclear facilities.

Aging behaviour of particular stainless-steels and NiFeCr alloy suitable for heat exchangers

This paper deals with the testing of three materials for special heat exchanger for short-time application. Mechanical and microstructural properties after aging at 650 and 850 °C were tested and analysed. The results will serve as an input data for the design and construction of plate heat exchanger.

Experimental and Numerical Investigation on a Water-Filled Cavity Natural Convection to Find the Proper Thermal Boundary Conditions for Simulations

ABSTRACTIn this study, the laminar natural convection flow inside a water-filled cavity with differentially heated vertical walls is investigated experimentally and numerically. Both of the walls are heated and cooled by two special heat exchangers that are attached to the walls and the rest are insulated. The main purpose of each test is to reach a uniform constant temperature on both of the heated and cooled walls. Early tests for an air-filled cavity showed that a uniform temperature on the walls is feasible, while a different trend was observed for a water-filled cavity with a nonuniform distribution of temperature. ANSYS FLUENT 15 employed four approaches in terms of boundary conditions for computational purposes. None of the three-dimensional (3D) and two-dimensional (2D) models of the cavity with a uniform wall temperature (the wall average temperature from the experiment) were suitable for predicting the Nusselt number. Therefore, it was essential to use the full model to properly predict the real distribution of temperature and Nusselt number on the walls. The 3D model of the cavity with a nonuniform wall temperature, which was borrowed from the experiment, also provided good results for the Nusselt number, but a measured temperature was still needed from the experiments. The 2D simulation's findings showed a weakness in properly capturing the streamlines for all ranges of Rayleigh numbers.

Helium liquefaction using a two stage Gifford McMahon Cryocooler

This paper describes helium liquefaction using a commercial cryocooler with 1.5 W cooling power at 4.2 K, equipped with heat exchangers for pre-cooling the incoming gas. No external cooling of helium gas or Joule-Thomson stages have been utilized. In this typical liquefaction system, helium gas at room temperature is initially pre-cooled by the cooling power available at the first stage of the cryocooler. Thereafter, it is cooled by the special heat exchanger (with large surface area) mounted on the second stage of the cryocooler. The gas when passing over the cold surface areas, gets condensed into a liquid and this enters the collecting vessel which is also mounted on the second stage of the cryocooler. A pressure gauge is connected to the collection vessel through a long narrow stainless pipe to monitor the vapour pressure of the liquid. When the liquid touches the end of this stainless steel pipe, thermoacoustic oscillations are observed on the pressure gauge, which is used for monitoring helium liquefaction. This paper describes the experimental setup and studies of liquefaction of helium by the cryocooler.

Experimental study on hybrid solar system fitted with special heat exchanger and flat reflectors

The hybrid photovoltaic/thermal (PV/T) solar water systems are not cost economic because of low concentration ratios, less combined efficiencies and non-standardisation of heat exchanger surfaces. The simple and effective technique used to increase performance of hybrid system must enhance available solar radiation on its surface for its working period. This can be accomplished by fitting flat reflectors to module side. This paper analyses performance of spiral flow heat exchanger of hybrid system with reflectors fixed to shorter sides of module. The results such as performance efficiencies of photovoltaic, thermal, and combined photovoltaic/thermal are compared for forced and natural circulation of water through heat exchanger of system. At peak PV point, natural flow system worked with combined efficiency of 56.60% and forced flow system worked with marginally high combined efficiency of 57.74%. The results of the study showed that for this system, thermo-siphon method if used can be an ideal solution for generation of electrical power and hot water.

Experimental study on hybrid solar system fitted with special heat exchanger and flat reflectors

The hybrid photovoltaic/thermal (PV/T) solar water systems are not cost economic because of low concentration ratios, less combined efficiencies and non-standardisation of heat exchanger surfaces. The simple and effective technique used to increase performance of hybrid system must enhance available solar radiation on its surface for its working period. This can be accomplished by fitting flat reflectors to module side. This paper analyses performance of spiral flow heat exchanger of hybrid system with reflectors fixed to shorter sides of module. The results such as performance efficiencies of photovoltaic, thermal, and combined photovoltaic/thermal are compared for forced and natural circulation of water through heat exchanger of system. At peak PV point, natural flow system worked with combined efficiency of 56.60% and forced flow system worked with marginally high combined efficiency of 57.74%. The results of the study showed that for this system, thermo-siphon method if used can be an ideal solution for generation of electrical power and hot water.

Experimental and theoretical investigations of special type coil heat exchanger with the nanofluid buffer layer

The paper presents the results of experimental and theoretical investigations of special type of coil heat exchanger. The tested device is equipped with three vertical coils and the temperature stratification system. Water is a heating medium in two coils. The refrigerant transferring the waste heat from air conditioning system is the heating medium in the third coil. The finned pipe of this coil has a double wall in which the annular buffer layer with nanofluid is mounted. Thermophysical properties of the applied water based Cu nanofluid cause the enhancement of heat transfer through the buffer layer. The paper presents thermal characteristics of the exchanger received on the basis of measurements performed on the industrial test stand. Measurements were conducted during the operation of the coil with refrigerant. Heat loss to the surroundings, distributions of water temperature in the storage tank, changes of water temperature in time and thermal power of the coil heat exchanger were obtained. The measurement results were compared with those received on the basis of theoretical analysis of the exchanger.

Experimental investigation on natural convection heat transfer characteristics of C-shape heating rods bundle used in PRHR HX

The heat transfer performance of Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) is of great importance for the efficient and safe removal of the residual heat in the AP1000 plant. The C-shape heating rods bundle is being used in the PRHR HX, yet the heat transfer characteristics of this special C-shape heat exchanger are not completely clear. The Westinghouse Electric Corporation has performed the PRHR HX experiment using three vertical tubes to validate the design of PRHR HX. However, the horizontal sections of the C-shaped bundle were neglected, and attentions were mainly focused on the local heat transfer effect of the vertical tubes. In the present work, an overall scaled-down IRWST and PRHR HX models are built to simulate the thermal hydraulic process in the residual heat removal accident. The natural convection heat transfer characteristics of both the overall IRWST model and the local C-shaped PRHR HX bundle model are researched in the first stage. For the overall IRWST model, the natural convection flow vectors are qualitatively measured by PIV technique. The velocity and temperature distributions are influenced mutually, then thermal stratification phenomenon develops in the overall IRWST. The existence of the “thermal interface” has great influences on the thermal stratification. For the local PRHR HX heat transfer, traditional natural convection correlations for both horizontal and vertical bundles are compared to the experimental data. For the vertical bundle of the PRHR HX model, a revised Rayleigh number is proposed as a criterion for the slender vertical rod bundles, when the vertical rod bundle outside flow cannot be treated as flat plate. Then revised correlations are proposed to predict the PRHR HX average heat transfer coefficient. Moreover, the analysis shows that the forced convection also plays an important role in enhancing the heat transfer effect of the PRHR HX model upper horizontal bundle, so the modified Numix is recommended for the PRHR HX upper horizontal bundle secondary heat transfer calculation.

Numerical simulation on the flow and heat transfer characteristics in the one-side heating helically coiled tubes

The circumferential heat flux often distributes non-uniformly in the heat exchange tubes, such as the coiled tubes water cooled wall in power plant boilers and the special heat exchangers used in the chemical reaction process. To study the thermal hydraulic characteristics of the flow in the helically coiled tube under different heating conditions, numerical simulations are conducted in tubes with uniform and one-side heating (7191⩽Re⩽47923, 0.983⩽Pr⩽6.992). The simulation results show that, compared with the case of uniform heating, the temperature difference between the outer side and inner side becomes more significant; The average volume fraction and the pressure drop of the two-phase flow are both lower. The local heat transfer coefficient mainly depends on the vapor distribution. The distribution of heat transfer coefficient around the cross-section is different in uniform and one-side heating tubes. The heat transfer coefficients around cross-section are found to display a “W” shape in uniform heating tube, but an “M” shape in one-side heating tube. Eventually, the differences of the average pressure drop and heat transfer coefficient under the two heating conditions are all lower than 15% within the present working conditions.

Experimental and numerical study of the thermal and hydrodynamic characteristics of laminar natural convective flow inside a rectangular cavity with water, ethylene glycol–water and air

Laminar natural convection in a rectangular cavity with three different heat transfer fluids: water, ethylene glycol (EG)–water and air were studied experimentally and numerically. The enclosure has a uniform aspect ratio (AR). The EG–water mixture is made up of 60% EG and 40% water. The main experiments aimed to reach proper thermal boundary conditions for the two differentially heated vertical walls of the cavity. Hence, two heating and cooling heat exchangers with water as the heat transfer fluid were attached to the cavity walls. All other walls were properly insulated. Early experiments revealed that it is hard for the heated and cooled walls to reach a uniform temperature when the cavity is filled with water or EG–water, while a uniform distribution of temperature was achieved when it is simply filled with air. Commercial computational fluid dynamics (CFD) software, ANSYS-FLUENT 15, simulated the entire setup to include two special heat exchangers and the cavity between them to investigate all the transport phenomena. The simulation results were in good agreement with measured data. The distortion of air flow is much higher than with the other two fluids. Water flow inside the cavity is flatter and a big circulation area was captured in the middle of the EG–water fluid flow. The local Nusselt number’s three-dimensional distribution was presented on the walls. When compared to other fluids, the impact of the adiabatic walls in the air flow cavity on the Nusselt number was found to be considerable. Eventually, the roles of energy equation terms were studied. Convective terms were noticeable when compared to thermal diffusion.

Modelling of a cross flow evaporator for CSP application: Analysis of the use of different two phase heat transfer and pressure drop correlations

Heat exchangers consisting of bundles of horizontal plain tubes with boiling on the shell side are widely used in industrial and energy systems applications. A recent particular specific interest for the use of this special heat exchanger is in connection with Concentrated Solar Power (CSP) applications. Heat transfer and pressure drop prediction methods are an important tool for design and modelling of diabatic, two-phase, shell-side flow over a horizontal plain tubes bundle for a vertical up-flow evaporator. With the objective of developing a model for a specific type of cross flow evaporator for a coil type steam generator specifically designed for solar applications, this paper analyzes the use of several heat transfer, void fraction and pressure drop correlations for the modelling the operation of such a type of steam generator.The paper after a brief review of the literature about the available correlations for the definition of two-phase flow heat transfer, void fraction and pressure drop in connection with the operation of steam generators, focuses attention on a comparison of the results obtained using several different models resulting by different combination of correlations. The influence on the analysis of the performance of the evaporator, their impact on significant design variables and the effective lifetime of critical components in different operating conditions, simulating the daily start-up procedures of the steam generator is evaluated. The importance of a good calibration of the model based on the comparison with some experimental data is recognised.

Coil Heat Exchanger with the Nanofluid Filled Buffer Layer

The paper presents the preliminary design of the special heat exchanger. The device under consideration is the kind of immersed coil heat exchangers. It consists of three vertical coils: two coils are standard, water is used as a heating medium; one coil is filled by the refrigerant R134a which transfers the waste heat from refrigeration and air conditioning system during the boiling processes. In order to prevent the possible refrigerant leakage, the special buffer layer filled with the nanofluid is mounted in the Freon coil. Thermophysical properties of the nanofluid cause the intensification of the heat transfer through the buffer layer and the same increase of the heat transfer rate. Calculations of thermal power were made. Correlations of heat transfer coefficients in curved tubes, pressure drop correlations for flow through helical coil tubes and correlations describing the heat transfer in the buffer layer, were applied. Results of the calculations indicate of the influence of of Freon coil on the exchanger heat transfer rate. Heat power of Freon coil is about 7 – 25% of water coil thermal power. Thus, the waste heat applied significantly increases the exchanger heat transfer rate.

COMPARISON OF MATHEMATICAL MODELS FOR HEAT EXCHANGERS OF UNCONVENTIONAL CHP UNITS

An unconventional CHP unit with a hot air engine is designed as the primary energy source with fuel in the form of biomass. The heat source is a furnace designed for combustion of biomass, whether in the form of wood logs or pellets. The transport of energy generated by the biomass combustion to the working medium of a hot-air engine is ensured by a special heat exchanger connected to this resource. The correct operation of the hot-air engine is largely dependent on an appropriate design of the exchanger. The paper deals with the calculation of the heat exchanger for the applications<br />mentioned, using criterion equations, and based on CFD simulations.

A composite cylindrical model and its application in analysis of thermal response and performance for energy pile

Energy pile is a special vertical heat exchanger with the advantage of saving land area for buried pipe in recent years. Based on the composite line source model and cylindrical model, this paper presents a composite cylindrical model, and the heat capacity of pile in the borehole in ground source heat pump system (GSHP) was considered in the model. The model can be used in energy pile with a large diameter. It was validated by comparing to a 3-D numerical model which had been compared with a measured data set. The thermal performance of various layout forms of heat exchangers in energy pile was analyzed. In addition, the model was applied to a project of thermal response test (TRT) to estimate the thermal property parameters of soil. The simulation results showed that the composite cylindrical model have a better agreement from start of the test with measurement data. The model gives a new simulation tool in analysis of performance and TRT for energy pile.

Indigenous development of a table top batch liquefaction for nitrogen using pulse tube crycooler

Cryogenic recondensation devices present considerable challenges towards condensing helium in systems such as MRI, NMR and SQUID cryostats etc. Normally refilling of cryogens is costly and also troublesome. To replace the standard refilling system, there is a need of small scale recondensing system. In a typical recondensation system, the evaporating gas is initially pre-cooled by the cooling power of the first stage cold head. Subsequently, it is passed on to the second stage cold head, where a special heat exchanger is mounted with large surface area for cooling. The gas when passing over this cold surface gets condensed into a liquid and returns back to the cryostat dewar. This paper presents the design and development of a simple experimental setup for batch liquefaction of nitrogen and is based on the two stage pulse tube Cryocooler already developed in our laboratory. The details of the system and the preliminary experimental results are presented here. This setup serves as a pre-runner for our subsequent development of re-condensation system for helium gas.

Calculation Method of Passive Residual Heat Removal Heat Exchanger and Numerical Simulation

The tube inside and outside heat transfer mechanism of Passive Residual Heat Removal Heat Exchanger (PRHR HX) was analyzed. The calculation method of this special heat exchanger under natural convection condition in In-containment Refueling Water Storage Tank (IRWST) was carried out. The single-tube coupling model three-dimensional natural circulation in the IRWST was simulated numerically using Fluent. The heat transfer and flow characteristics of the fluid in IRWST were obtained. The comparison of the results between theoretical arithmetic and numerical simulation showed that the theoretical calculation method is suitable for the heat transfer calculation of PRHR HX.

G-Functions for multiple interacting pile heat exchangers

Pile heat exchangers – where heat transfer pipes are cast into the building piled foundations – offer an opportunity to use ground energy systems without the additional construction costs related to the provision of special purpose heat exchangers. However, analysis methods for pile heat exchangers are still under development. In particular there is an absence of available methods and guidance for the amount of thermal interaction that may occur between adjacent pile heat exchangers and the corresponding reduction in available energy that this will cause. This is of particular importance as the locations of foundation piles are controlled by the structural demands of the building and cannot be optimised with respect to the thermal analysis. This paper presents a method for deriving G-functions for use with multiple pile heat exchangers. Example functions illustrate the primary importance of pile spacing in controlling available energy, followed by the number of piles within any given arrangement. Significantly it was found that the internal thermal behaviour of a pile is not influenced appreciably by adjacent piles.

An innovative pool with a passive heat removal system

Heat removal systems are of primary importance in several industrial processes. As heat sink, a water pool or atmospheric air may be selected. The first solution takes advantage of high heat transfer coefficient with water but it requires active systems to maintain a constant water level; the second solution takes benefit from the unlimited heat removal capacity by air, but it requires a larger heat exchanger to compensate the lower heat transfer coefficient. In NPPs (nuclear power plants) during a nuclear reactor shutdown, as well as in some chemical plants to control runaway reactions, it is possible to use an innovative heat sink that joins the advantages of the two previous solutions. This solution is based on a special heat exchanger submerged in a water pool designed so that when heat removal is requested, active systems are not required to maintain the water level; due to the special design, when the pool is empty, atmospheric air becomes the only heat sink. The special heat exchanger design allows to have a heat exchanger without being oversized and to have a system able to operate for unlimited period without external interventions. This innovative system provides an economic advantage as well as enhanced safety features.

Heat exchanger leakage problem location

Recent compact heat exchangers are very often assembled from numerous parts joined together to separate heat transfer fluids and to form the required heat exchanger arrangement. Therefore, the leak tightness is very important property of the compact heat exchangers. Although, the compact heat exchangers have been produced for many years, there are still technological problems associated with manufacturing of the ideal connection between the individual parts, mainly encountered with special purpose heat exchangers, e.g. gas turbine recuperators. This paper describes a procedure used to identify the leakage location inside the prime surface gas turbine recuperator. For this purpose, an analytical model of the leaky gas turbine recuperator was created to assess its performance. The results obtained are compared with the experimental data which were acquired during the recuperator thermal performance analysis. The differences between these two data sets are used to indicate possible leakage areas.

Spacecraft “Foton-M” in-flight thermal conditions

The problem of thermal conditions aboard the “Foton-M” spacecraft during its orbital flight is under consideration in this paper. The problem is very acute for performing microgravity experiments onboard of the orbital platform, because on one hand, many experiments need a definite temperature range to be performed, and on the other hand all electrical devices aboard radiate heat. To avoid uncontrolled heating of the environment special heat exchangers are used. To transport heat from different places of the capsule to heat exchanger special fans are installed given definite orientation. All the heat exchange facilities should be designed in advance being adjusted to current capsule loading and heat radiation by equipment. Thus special tools are needed predicting the capsule thermal conditions being function of equipment placement. The present paper introduces a new developed prognostic mathematical model able to forecast temperature distribution inside the capsule with account of fan induced air flows, thermal irradiation by scientific equipment and heat losses due to cooling system.