Nano-particle deposition in the presence of electric field

The dispersion and deposition of nano-particles in laminar flows in the presence of an electric field were studied. The Eulerian-Lagrangian particle tracking method was used to simulate nano-particle motions under the one-way coupling assumption. For nano-particles in the size range of 5–200 nm, in addition to the Brownian excitation, the electrostatic and gravitational forces were included in the analysis. Different charging mechanisms including field and diffusion charging as well as the Boltzmann charge distributions were investigated. The simulation methodology was first validated for Brownian and electrostatic forces. For the combined field and diffusion charging, the simulation results showed that in the presence of an electric field of 10 kV/m, the electrostatic force dominates the Brownian effects. However, when the electric field was 1 kV/m, the Brownian motion strongly affected the particle dispersion and deposition processes. For the electric field intensity of 1 kV/m, for 10 nm and 100 nm particles, the deposition efficiencies for the combined effects of electrostatic and Brownian motion were, respectively, about 27% and 161.2% higher than the case in the absence of electric field. Furthermore, particles with the Boltzmann charge distribution had the maximum deposition for 20 nm particles

Impact of variable fluid properties on forced convection of Fe3O4/CNT/water hybrid nanofluid in a double-pipe mini-channel heat exchanger

The objective of this study is to assess the hydrothermal performance of a non-Newtonian hybrid nanofluid with temperature-dependent thermal conductivity and viscosity compared with a Newtonian hybrid nanofluid with constant thermophysical properties. A counter-current double-pipe mini-channel heat exchanger is studied to analyze the effects of the hybrid nanofluid. The nanofluid is employed as the coolant in the tube side, while the hot water flows in the annulus side. Two different nanoparticles including tetramethylammonium hydroxide-coated Fe3O4 (magnetite) nanoparticles and gum arabic-coated carbon nanotubes are used to prepare the water-based hybrid nanofluid. The results demonstrated that the non-Newtonian hybrid nanofluid always has a higher heat transfer rate, overall heat transfer coefficient, and effectiveness than those of the Newtonian hybrid nanofluid, while the opposite is true for the pressure drop, pumping power, and performance evaluation criterion. Supposing that the Fe3O4-carbon nanotube/water hybrid nanofluid is a Newtonian fluid with constant thermal conductivity and viscosity, there leads to large error in the computation of pressure drop (1.5–9.71%), pumping power (1.5–9.71%), and performance evaluation criterion (18.24–19.60%), whereas the errors in the computation of heat transfer rate, overall heat transfer coefficient, and effectiveness are not considerable (less than 2.91%)

Melting and solidification characteristics of a double-pipe latent heat storage system with sinusoidal wavy channels embedded in a porous medium

The aim of this investigation is to explore the combined effects of porous medium and surface waviness on the melting and solidification of PCM inside a vertical double-pipe latent heat storage (LHTES) system. The results are compared with the cases of smooth channels and pure PCM. In the system, water is passed through the inner tube while composite PCM is placed in the annulus side. Different effective parameters including wavelength and wave amplitude of the sinusoidal wavy channels, porosity and pore size of the porous structure, Reynolds number and inlet temperature of water are examined to find the optimum geometric as well as operating conditions in both melting/solidification processes. The results show that utilizing both the high conductive porous structure and wavy channel reduces the melting/solidification times significantly. For the best case, the melting and solidification times of PCM reduce by 91.4% and 96.7%, respectively, compared with the smooth channels pure PCM system. The average rate of transferred heat for the wavy channel composite PCM are 10.4 and 18.9 times that for the smooth channel pure PCM case. Comparing with the pure PCM system, the presence of copper foam reduces the effect of channel waviness significantly for both melting/solidification processes

Free convection heat transfer and entropy generation analysis of water-Fe 3 O 4 /CNT hybrid nanofluid in a concentric annulus

© 2018, Emerald Publishing Limited. Purpose: This paper aims to numerically investigate the heat transfer and entropy generation characteristics of water-based hybrid nanofluid in natural convection flow inside a concentric horizontal annulus. Design/methodology/approach: The hybrid nanofluid is prepared by suspending tetramethylammonium hydroxide-coated Fe 3 O 4 (magnetite) nanoparticles and gum arabic (GA)-coated carbon nanotubes (CNTs) in water. The effects of nanoparticle volume concentration and Rayleigh number on the streamlines, isotherms, average Nusselt number and the thermal, frictional and total entropy generation rates are investigated comprehensively. Findings: Results show the advantageous effect of hybrid nanofluid on the average Nusselt number. Furthermore, the study of entropy generation shows the increment of both frictional and thermal entropy generation rates by increasing Fe 3 O 4 and CNT concentrations at various Rayleigh numbers. Increasing Rayleigh number from 103 to 105, at Fe 3 O 4 concentration of 0.9 per cent and CNT concentration of 1.35 per cent, increases the average Nusselt number, thermal entropy generation rate and frictional entropy generation rate by 224.95, 224.65 and 155.25 per cent, respectively. Moreover, increasing the Fe 3 O 4 concentration from 0.5 to 0.9 per cent, at Rayleigh number of 105 and CNT concentration of 1.35 per cent, intensifies the average Nusselt number, thermal entropy generation rate and frictional entropy generation rate by 18.36, 22.78 and 72.7 per cent, respectively. Originality/value: To the best knowledge of the authors, there are not any archival publications considering the detailed behaviour of the natural convective heat transfer and entropy generation of hybrid nanofluid in a concentric annulus

Recent Developments of Combined Heat Pump and Organic Rankine Cycle Energy Systems for Buildings

To develop efficient and lower emission heating and cooling systems, this book chapter focuses on interests for the innovative combination of a heat pump (HP) and organic Rankine cycle (ORC) for building applications. In this state-of-the-art survey, the potentials and advantages of combined HP-ORC systems have been investigated and discussed. Past works have examined various combinations, comprising indirectly-combined as series and parallel, directly-combined units, as well as reversible combination configurations. Following describing such arrangements, their performance is discussed. Considerations for optimising the overall architecture of these combined energy systems are pinpointed using these same sources, taking into account heat source and sink selection, expander/compressor units, selection of working fluids, control strategies, operating temperatures, thermal energy storage and managing more variable seasonal temperatures. Furthermore, experimental works present further functional problems and matters needing additional research, and assist to emphasise experimental techniques that can be utilised in this field of research. Finally, from the studies surveyed, some areas for future research were recommended

Numerical study of a multiple-segment metal foam-PCM latent heat storage unit: Effect of porosity, pore density and location of heat source

This study numerically investigates the performance of the melting process for a PCM based heat storage system under the effect of different variables in a vertical container with a copper metal foam. Different cases were studied and compared including the effects of variable porosities and pore densities, non-equilibrium porous medium model, a multiple-segment metal foam case and different heater locations in the system on the liquid fraction and temperature as presented by contour plots and diagrams. The results show high performance for the copper foam-PCM unit compared with on its own PCM, for reducing the melting time by almost 85%. By changing the location of constant temperature heater from the bottom to the side and top surface, the melting time decreases by 70.5% and 4.7%, respectively. By using a multiple-segment porous system, the melting time reduces by 3.5% compared with the case of uniform porosity. Furthermore, the more accurate non-equilibrium numerical model shows a 7.4% difference in the melting time compared with the equilibrium model. This study optimises the design to improve practical application performance and to reduce waste energy

Energy and exergy analysis of two novel hybrid solar photovoltaic geothermal energy systems incorporating a building integrated photovoltaic thermal system and an earth air heat exchanger system

In this paper, two novel configurations of the building integrated photovoltaic thermal (BIPVT)-compound earth-air heat exchanger (EAHE) system are proposed. Both the configurations operate in two modes, namely heating and cooling modes. In the heating mode of the configuration A, the cold outdoor air is twice preheated by passing through the EAHE and BIPVT systems. In the cooling mode of the configuration A, the hot outdoor air is precooled by flowing inside the EAHE system and the PV modules are cooled using the building exhaust air. The cooling mode of the configuration B is similar to the configuration A, while in the heating mode of the configuration B, the outdoor air first enters the BIPVT collector and then passes through the EAHE system. The energetic and exergetic performances of the configurations are investigated for climatic conditions of Kermanshah, Iran. In addition, the impacts of length, width, and depth of air duct located underneath the PV panels, air mass flow rate, length and inner diameter of the pipe of EAHE system on the annual average energetic and exergetic aspects of the best configuration of the BIPVT-EAHE system are evaluated. The outcomes revealed that the annual rate of thermal energy, electrical energy, and thermal exergy captured from the configuration A are respectively 3499.59, 5908.19, and 55.59 kWh, while these values for the configuration B are respectively 3468.16, 5969.87, and 51.76 kWh. In addition, it was found that the configuration A has superior energetic performance than the configuration B, while the overall exergetic performance of the configuration B is higher than the configuration A. Furthermore, it was depicted that both the energetic and exergetic performances of the suggested configurations intensify by augmenting the duct length, duct width, and tube diameter whereas they decline with an increase in the air mass flow rate and duct depth

Numerical modelling of phase change material melting process embedded in porous media: Effect of heat storage size

The aim of this paper is to study the influence of enclosure size in latent heat thermal energy storage systems embedded in a porous medium for domestic usage of latent heat thermal energy storage heat exchangers. A 2-D rectangular enclosure is considered as the computational domain to study the heat transfer improvement for a phase change material embedded in a copper foam considering a constant heat flux from the bottom surface. Different dimensions of the composite system are examined compared with a system without a porous medium. The thermal non-equilibrium model with enthalpy-porosity method is employed for the effects of porous medium and phase change in the governing equations, respectively. The phase change material liquid fraction, temperature, velocity, stream lines and the rate of heat transfer are studied. The presence of a porous medium increases the heat transfer significantly, but the improvement in melting performance is strongly related to the system's dimensions. For the dimensions of 200 × 100 mm (W × H), the melting time of porous-phase change material with the porosity of 95% is reduced by 17% compared with phase change material-only system. For the same storage volume and total amount of thermal energy added, the melting time is lower for the system with a lower height, especially for the phase change material-only system due to a higher area of the input heat. The non-dimensional analysis results in curve-fitting correlations between the liquid fraction and Fo.Ste.Ra−0.02 for rectangular latent heat thermal energy storage systems for both phase change material-only and composite-phase change material systems within the parameter range of 1.16 less than less than Lf less than 1 and 0 less than Fo.Ste.Ra−0.02 less than 0.57. Over a range of system's volume, heat flux and surface area of the input heat flux, the benefit of composite phase change material is variable and, in some cases, is negligible compared with the phase change material-only system

Discharge of a composite metal foam/phase change material to air heat exchanger for a domestic thermal storage unit

This paper evaluates the discharging mechanism in a PCM (phase change material) to air heat exchanger for the purpose of space heating using a composite of copper foam and PCM. The composite system is modelled with both 2-D and 3-D computational fluid dynamics approach for different inlet air temperatures to consider the effect of room temperature using the thermal non-equilibrium model for the porous medium compared with the thermal equilibrium one. The results show the significant advantages of composite heat exchanger compared with a PCM only case. For the inlet air temperature of 22 °C, the composite unit is solidified in 43% shorter time with 73% higher heat retrieval rate compared with that for the PCM only. After 10 h, the temperature variation between the inlet and outlet of the air channels for latent heat storage heat exchanger system with the composite system is 41 °C and 34 °C for the inlet air temperatures of 0 °C and 22 °C, respectively, while it is 33 °C and 29 °C for the system with PCM only. This study show the possible usage of PCMs in the energy storage heaters by introducing metal foams which is not possible using PCM only alternatives

Transient simulation of finned heat sinks embedded with PCM for electronics cooling

This paper reports the two-dimensional (2D) transient numerical simulation of a phase change material (PCM) based finned heat sink to investigate the heat transfer performance for passive cooling of electronic devices. The finned heat sinks of 2 mm and 3 mm fin thickness are employed with a constant fin volume fraction of 9%, acting as thermal conductivity enhancer (TCE). The n-eicosane is employed as a PCM inside the heat sink to store the heat generated from the electronic device applied at the heat sink base. Transient numerical simulations are performed using finite-volume-method and conjugate heat transfer and melting/solidification phenomenon are investigated by applying various power levels. The numerical results show that the employed PCM with low temperature keeps the heat sink base temperature in lower limits and uniform melting is observed inside the finned heat sink. With the increase of heating power level, the PCM melting time is decreased for fin thickness heat sinks. By increasing the power level from 4 to 6 W, for the case of 3 mm fin thickness, the melting time increases by 6.63%, 3.59% and 1.90% by 3 mm fin thickness heat sink, compared to the 2 mm fin thickness heat sink. The developed equations of liquid fraction and modified Nusselt number are obtained as function of modified Fourier number, Stefan number, and Rayleigh number which provide guidelines for generalizing the thermal performance of PCM based finned heat sinks