Numerical Simulation of Sub-cooled Boiling Flow with Fouling Deposited inside Channels

This document is the Accepted Manuscript version of the following article: X. Liu, X. Zhang, T. Lu, K. Mahkamov, H. Wu, and M. Mirzaeian 'Numerical simulation of sub-cooled boiling flow with fouling deposited inside channels', Applied Thermal Engineering, Vol. 203, pp. 434-442, June 2016. The version of record is available online at doi: https://doi.org/10.1016/j.applthermaleng.2016.04.041. © 2016 Elsevier Ltd. All rights reserved.In this article, a numerical simulation has been performed to investigate the sub-cooled boiling flow in axisymmetric channels using the two-phase particle model. The equivalent diameter of the channel is 4.38 mm with 365.7 cm in length. The fouling deposited layer is filled with subsequent two-thirds of the flow channel. The internal surface of the channel is covered by a fouling deposit layer with a thickness ranging from 0.225 mm to 1.55 mm. Uniform heat flux of 29267.6 W/m2 is applied on the heated wall. Validation of the CFD model is carried out through comparison with open published experimental data and a close agreement is achieved. A new parameter, Security factor, is introduced and defined in the current study. Numerical results show that the developed two-phase particle model could well predict the water-steam two-phase change flow. The Nusselt number in the fouling region without fouling deposited could be 50 times higher than that with fouling layer. The heat transfer performance of the channel with thickness of 0.225 mm fouling deposit layer is 5 times larger than that with thickness of 1.55 mm fouling deposit layer. It is also found that the inlet velocity has significant impact on the boiling and total pressure drops along the channel.Peer reviewe

Optical Fibre Sensors for Monitoring Phase Transitions in Phase Changing Materials

A platinum coated singlemode-multimode (SM) structure is investigated in this paper as an optical fibre sensor (OFS) to monitor the phase transition of a phase change material (PCM). Paraffin wax has been used as an example to demonstrate the sensor\u27s performance and operation. Most materials have the same temperature but different thermal energy levels during the phase change process, therefore, sole dependency on temperature measurement may lead to an incorrect estimation of the stored energy in PCM. The output spectrum of the reflected light from the OFS is very sensitive to the bend introduced by the PCM where both liquid and solid states exist during the phase transition. The measurement of strain experienced by the OFS during the phase change of the PCM is utilized for identifying the phase transition of paraffin wax between the solid and liquid states. The experimental results presented in this paper show that the OFS with a shorter multimode fibre section has better performance for monitoring the phase transition of paraffin wax with a measured phase transition temperature range of 41.5 °C–57.7 °C for the SM based OFS with a 5 mm long multimode fibre section

Black box modelling of a latent heat thermal energy storage system coupled with heat pipes

This paper presents black box models to represent a LHTESS (Latent Heat Thermal Energy Storage System) coupled with heat pipes, aimed at increasing the storage performance and at decreasing the time of charging/discharging. The presented storage system is part of a micro solar CHP plant and the developed model is intended to be used in the simulation tool of the overall system, thus it has to be accurate but also fast computing. Black box data driven models are considered, trained by means of numerical data obtained from a white box detailed model of the LHTESS and heat pipes system. A year round simulation of the system during its normal operation within the micro solar CHP plant is used as dataset. Then the black box models are trained and finally validated on these data. Results show the need for a black box model that can take into account the different seasonal performance of the LHTESS. In this analysis the best fit was achieved by means of Random Forest models with an accuracy higher than 90%

An axisymmetric computational fluid dynamics approach to the analysis of the working process of a solar stirling engine

The use of computational fluid dynamics (CFD) models significantly extends the capabilities for the detailed analysis of the complex heat transfer and gas dynamic processes that occur in the internal gas circuit of a Stirling engine by more accurately predicting the engine's performance. This accurate data on operational characteristics of the engine can then contribute to more precise calculations of the dimensions of a parabolic concentrator in a dish/Stirling engine installation. In this paper a successful axisymmetric CFD simulation of a solar "V"-type Stirling engine is described for the first time. The standard - turbulence model, with a moving mesh to reflect the reciprocating motion of the pistons, has been employed for the analysis of the engine's working process. The gas temperature and pressure distributions and velocity fields in the internal gas circuit of the machine have been obtained and the pressure-volume diagrams have been calculated. Comparison of the numerical results produced from the axisymmetric CFD simulation of the engine's working process with those computed with the use of second-order mathematical analysis shows that there are considerable differences. In particular, analysis of the data obtained indicates that the gas temperature in the compression space depends on the location in the cylinder for the given moment in the cycle and it may differ substantially from being harmonic in time

Design improvements to a biomass stirling engine using mathematical analysis and 3D CFD modelling

A prototype of a biomass Stirling engine was developed and manufactured by an industrial company prior to the numerical investigations described in this paper. Dimensions and performance of the prototype had originally been estimated using the company's own simplified "first-order" mathematical model of the engine's working cycle. The manufactured engine was experimentally tested, and the results demonstrated that the power output from the machine was far less than expected. To understand what caused the engine's low operational characteristics and to predict how to refine the design, more advanced numerical investigations of the working process of the engine were performed. This utilized a "second-order" type 5 control volumes model, together with three-dimensional computational fluid dynamics modeling. As an outcome of this study, several recommendations on to how alter the prototype's design were forthcoming, which, in practice, allow significant improvements in the engine's performance

Development of 'low-tech' solar thermal water pumps for use in developing countries

Solar water pumps, based on electro-mechanical pumps powered by PV arrays, are commonly used and commercially available. However, one of the difficulties for their wider application in developing countries, where there is a high average insolation, is their relatively excessive cost. This arises mainly due to the high cost of the PV elements. Hence, this paper describes some developmental work and results of experimental tests on "low-tech" solar thermal water pumps which were built on the basis of Stirling engines with fluid pistons coupled to flat-plate solar collectors. Temperatures and pressures in the cycle are comparatively low, thus cheap design materials, such as glass and plastic, and a simple technology, available in the majority of mechanical workshops, can be used for their manufacture and consequently reduce their cost. Several design modifications of the above solar thermal water pumps have been developed and tested. The results obtained demonstrate that existing installations can be effectively applied for water pumping with a dynamic head which varies between 2-5 m. Furthermore, data from experimental tests shows that the pulsating motion of water in channels of the flat-plate solar collectors increases the collector's efficiency by approximately 8-10%, which is a considerable advantage when a pump is used as part of a house solar heating system

Solar thermal water pumps : a preliminary analysis of the working process

Solar thermal water pumps are low cost and low maintenance devices with a pumping capacity of 0.2-1 m3/hour at a dynamic head of 1.5–5 m. The working fluid in the thermodynamic cycle is an air-steam mixture. In this paper we suggest a simple mathematical model to numerically simulate the internal processes in such a pump and determine the performance and physical dimensions of a preliminary design. The proposed mathematical model has been calibrated against experimental data and it provides the numerical simulation of the processes which occur in the cycle within an acceptable degree of accuracy for engineering purposes. The results of the analysis show that the performance of the solar water pump is mainly determined by the "steam" fraction of the cycle. The power of the solar thermal water pump increases with an increase in the maximum temperature in the cycle, while the indicated efficiency reduces because of the increase in the heat loss due to water vaporization and condensation processes

Integration and optimisation of high-penetration Hybrid Renewable Energy Systems for fulfilling electrical and thermal demand for off-grid communities

The recent steep decline in the cost of PV panels and wind turbines provides an opportunity to utilise Hybrid Renewable Energy Systems (HRES) to fulfil thermal loads as well as the electrical demand. Two coupled system architectures are proposed and studied along with the base case of uncoupled architecture for comparison. The coupled systems include wind turbines, PV panels, battery bank, diesel generators (DG) or diesel-based Combined Heat and Power unit (CHP), boiler and thermal storage tank. Adopting a full factorial design of the experiment approach for each architecture, the optimum system is determined for a variety of combinations of the major influencing factors, namely, renewable resource intensity, thermal-to-electrical demand ratio and price of diesel fuel. Statistical analysis is conducted to investigate the effects of each factor on the cost of the system. The optimisation and statistical analysis show that, based on current prevailing components prices, an integrated system, which utilises excess electricity for fulfilling thermal loads, is cheaper than the base case with or without CHP unit