Analysis of heat and mass transfer by CFD for performance enhancement in direct contact membrane distillation

A comprehensive analysis on the dominant effects for heat and mass transfer in the direct contact membrane distillation (DCMD) process has been performed with the aid of computational fluid dynamics (CFD) simulations for hollow fiber modules without and with annular baffles attached to the shell wall. Potential enhancement strategies under different circumstances have been investigated. Numerical simulations were carried out to investigate the effect of the MD intrinsic mass-transfer coefficient of the membrane (C) on the performance enhancement for both non-baffled and baffled modules. It was found that the temperature polarization coefficient (TPC) decreases significantly with increasing C value regardless of the existence of baffles, signifying a loss of overall driving force. However, the higher C compensated for this and the mass flux showed an increasing trend. A membrane with a lower C value was found to be less vulnerable to the TP effect. In this case, the introduction of turbulence aids such as baffles did not show substantial effect to improve system performance. In contrast, introducing baffles into the module can greatly enhance the mass flux and the TPC for a membrane with a high C value, where the main heat-transfer resistance is determined by the fluid side boundary layers. The effect of operating temperature on heat and mass transfer in the MD process was also studied with a membrane of a lower C value (2.0 × 10−7 kg m−2 s−1 Pa−1). Although the TPC generally decreased with increasing operating temperatures, the mass flux Nm increased significantly when operating temperature increased. A baffled module showed a more significant improvement than a non-baffle module at a higher temperature. Moreover, it was confirmed that higher operating temperatures are preferable for a substantial improvement in the heat/mass transfer as well as MD thermal efficiency, even with a relatively small transmembrane temperature difference of 10 K.Accepted versio

Numerical Investigation of the Water/Alumina Nanofluid within a Microchannel with Baffles

The study of heat transfer phenomenon in microchannels has attracted researchers’ attention as they have many advantages in the cooling of electronic components. In this numerical study, the effect of adding alumina nanoparticles to the water flow through a microchannel with some baffles embedded on the top and bottom walls is numerically discussed by using ANSYS Fluent software. The several cases including the effect of various volume fraction of nanoparticles (2, 4, 6, and 10%), Reynolds number of the inlet flow (10, 20, 30, 40, and 50), and the number of baffles and their heights on the heat transfer phenomena are investigated. The local Nusselt number, the average outlet temperature, and the streamlines are presented for representing the results. The results show that increasing the Reynolds number decreases the average outlet temperature. Moreover, the increase in the number of baffles causes an increase in the average outlet temperature since the formation of vorticities just behind of each baffle and results in a large heat transfer rate. As the baffles height increase, the strength and the area of the vortices increase and hence the heat transfer rate increases. However, an increase in the volume fraction of the nanoparticle increases the average outlet temperature which is due to the increase in conduction heat transfer of nanoflui

Numerical parametric study of a cooling system for an LNG storage tank

International audienceThe study of the cooling system of a Liquefied Natural Gas (LNG) storage tank is vital for the safety of the installation. The objective of this paper is to develop cooling baffles capable of reducing the heat gain from the environment leading to a loss of LNG quantity, keeping the Boil-Off Gas (BOG) under control. For this purpose, a specific code based on the finite volume method was developed to improve our knowledge of the hydrodynamic and thermal behaviors of LNG in the cylindrical tank. In addition, the effect of the number, position and dimension of the baffles on the flow structure of LNG were determined. The obtained results indicated that the location of the baffles at the top of the tank nearby the vicinity of the wall would yield a better cooling of the LNG. Moreover, we emphasized that a number of six baffles would give rise to a better heat transfer. For a design purpose, the Nusselt numbers on the lateral surface and on the baffles have been correlated as functions of Rayleigh and baffle numbers

Experimental studies on hydrodynamic behaviour of flow through a tube with Triangular Wavy Tapes (TWT)

The present project work includes the introduction of TWT as inserts as passive augmentation device for tube side liquid flow.The effect of turbulence on friction factor was measured and compared with the values for smooth tube.The effect of baffles was also taken into account and again a comparative study was made on the basis of varying the baffle spacing.All the results and readings were compared with the standard data from the smooth tube.Whenever inserts are used for the heat transfer enhancement,both the heat transfer rate and the pressure drop increse.This increse in pressure drop increases the pumping cost.Thus it is highly essential not to allow the pressure drop to go beyond a specified value while going for heat transfer enhancement techniques using inserts. Experimental work on hydrodynamic behaviour using TWT is executed.Inserts when placed in the path of the flow of the liquid,both the heat transfer rate and the pressure drop increase, because of increased degree of turbulence created.The present study includes the determination of friction factoe for various TWT and its ten modifications.In the beginning,we conducted the experiment without any insert to get the value for plain tube and thereafter the experiment was repeated with TWT without any baffles and with baffleswith varying baffle spacing.The results of TWT without any baffles and with baffles with varying baffle spacing have been comparedwith values for the smooth tube.It was also observed that with an increase in Reynolds number (Re),the friction factor decreases.The highest value for fa/f0 was found to be around 12

Experimental studies on heat transfer augmenatation using galvanised iron wire insert with and without baffles

The present project work on “Experimental Studies on Heat Transfer Augmentation Using Galvanised Iron Wire Insert With and Without Baffles” was undertaken with a view to increase the effective thermal performance of heat exchangers. Every commercial, industrial and domestic application, where conversion or utilization of energy is involved, requires a heat exchange process. In the present project work, the effect of addition of GI wire insert with and without baffles, in the flow path of liquid was investigated. This project deals with the using four Twisted Galvanised Iron (GI) wires with and without baffles, as passive augmentation device. By introduction of these inserts in the flow path of liquid in the inner tube of the heat exchanger the effect of turbulence on Nusselt number was observed. It was compared with the values for smooth tube. The effect of baffles made of tin was also taken into account and a comparative study was made on the basis of varying baffle space. All the results and readings were compared with the standard data from the smooth tube. The Nusselt number was found to increase with decreasing baffle space. The 4 wire insert with baffle space 6cm was found to be the most efficient among all the configurations used