CODEN (USA): IJCRGG ISSN: 0974-4290 Vol.9, No.01 pp 15-22, 2016

Abstract: Simulations using the k–epsilon model have been carried out to investigate the fluid flow and heat transfer characteristics in the Heat Exchanger with copper porous media. In designing heat exchanger, copper ridges are made on the surface in order to enhance the heat transfer. The parameters studied include the Reynolds number (Re<2000), pressure drop, temperature, thickness of the porous media used by maintaining the porosity e = 0.8. The comparison analysis is done between the computational work and existing heat exchanger with same boundary condition. Results show that newlydesigned heat exchanger enhance the heat transfer up to 150C. Keywords : Heat Exchanger; Finite volume; Heat Transfer; Pressure Drop; Temperature; Simulation; k-epsilon model; copper ridges.

Heat exchangers are mostly used in automobiles and other application like space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment1. Where fluids are separated by a solid assembly like wall to anticipate bond or they may be in direct contact. Heat exchangers are made out of highly thermally conductive materials for reduction of heat. There are various types of heat exchangers (about 12 types) and details of all the heat exchangers has been collected for study purposes. A critical investigation on every method has been made and the problems found in the above-stated method are size, efficiency & cost.

The development of electronic industry and its trend toward miniaturization and high speed operating processes requires higher performance and small scale cooling systems. The miniaturized Heat exchanger has turned into an essential issue in the connection of cutting edge technologies. Its advancement is displayed as an inspiration to the subject of this paper. In this subject, there are two vital aspects of engineering: a thought radiating to fulfill some need, and the theoretical and computational encapsulation of the thought, keeping in mind the end goal to outline a productive Heat exchanger hardware, one must know the points of interest of both flow and heat transfer attributes in the equipment. So such point by point flow and heat attributes in of an assembly can be investigated theoretical and computational way by solving a set of governing equations based on the principles. Porous media is chosen for developing a new compact heat exchanger. The porous media models had been used in the one to many advanced engineering fields which include flows through filter papers, packed beds, perforated plates, Air Filters, tube banks, and distributors2.

The majority of the earlier studies dealing with heat and mass transfer in porous media are mostly based on the empirical correlations suggested by earlier researchers2-15, which mainly concentrates in the applications based on adsorption, stripping and distillation, geothermal operation3. Research works which use the porous media concept for design and fabrication of Heat exchanger using Porous Copper were not found in the known literature.

Flow through porous media has been considered as the complex element until the time that Darcy’s distribution is proposed2. The expansion of Darcy Law was intended by Dupuis16,17, Forchheimer18. Their works characterize the fluid flow over a porous medium. From that point, a progression of upgrades has been made, one of which was representing temperature variations in the fluid19. These temperature variations were later connected to the thickness of the porous media8. A survey of the historical backdrop of the investigation of fluid flow through porous mediawas done21-22 . The generally acknowledged mathematical equation in investigating flow through porous media shown in Eq. 1 which superintend the pressure drop(∆P) of a fluid through a porous medium.

1Further developments in porous media include convective heat transfer8-20. Investigation in heat transfer through porous media was done by considered a complete profile as simple uniform cubic unit cell and analyzed fluid velocity profile and rate of cooling13. Better explanatory models were presented in convective heat transfer by introducing extended Darcy model19. Different theoretical models have been proposed to explain different variables, for example, wall effect20, variable porosity23etc. The larger part of these studies that are secured by considering circular porous media, which have porosities (e) in the range 0:3–0:6. This design is known as a packed bed24.

The packed bed is basically fabricated either by using spherical particulates or non-spherical particles25. Open cell foams are one kind of a non-special porous media. The arrangement of non-spherical particle opens itself to a wide assortment of conceivable applications26.The open-cell metal porous media structure have attractive characteristics to be a heat exchanger, i.e. a Conducting solid surface, solid–fluid interface and high surface area. Contingent upon the specific open cell metal porous media arrangement, its particular surface area of the packed structure varies from 500- 10,000m^225. The composite can be fabricated from the materials with good thermal conductivity constant, for example, aluminum or copper which, just by its vicinity in a static fluid, significantly increases the thermal conductivity of the fluid too8. The conductivity of the solid–fluid framework can be anticipated by Eq. (2) was first explained in the 2-D conduction model27.

2Taking structure of the porous media into consideration, an enhanced model was explained considering porous media as a 3D unit cell10, where author analyzed both thermal conductivity and fluid flow. Many scientists had continued exploring the Packed bed and as of now there exist a data bank of theoretical3 -7and experimental models7-15, 25based on the size, shape, orientation of structure of the porous media. The standard governing equation with boundary conditions for flow and heat transfer in open cell porous media27. On account of open-cell metal porous media, the numerical models had constrained achievement14, and the experimentation to check these models is limited, especially to the coolant, which is common air. In cooling hardware which produce a lot of overabundance heat, a fluid coolant is preferred. In the perspective of these prerequisites, tests utilizing a constrained fluid coolant are required not just to examine the possibility of utilizing open-cell porous media as Heat exchangers, additionally to give a premise against which are theoretically analyzed.

Most of the applications which are based on the porous media are manufactured by anodizing required material wafer into electrolytes28 (like Hydrofluoric acid/ethanol/water). The thickness of porous media (d) and porosity (p) are the driving parameters helps to determine the concentration electrolyte solution29.In the recent past, a new methodology was proposed in fabricating the porous media using powder metallurgy15. Fabricating the porous media by slip casting process gives an advantage in deciding the porosity.

The following design and simulation premises are considered based on the literature survey and the requirements of the heat exchanger are;

III Design and Simulation of Heat Exchanger:

Figure 1. Heat Exchanger fitted with Porous Media and ridges

In the main goal of this work is to design a compact Heat exchanger with Copper porous media fitted at the outer surface and copper ridges are made on the surface as shown in figure 1.The porous on the outer surface is considered as heat pipe31. Heat pipe plays significant role in designing the porous media heat exchanger and it is made out of a fixed, emptied vessel in which the internal surfaces are filled with a Cu porous media and loaded with enough working liquid to immerse the porous media32. Heat is absorbed from the working fluid will evaporate the fluid in the heat exchanger and set out as a vapor. This vapor will move to the cooler region of the vessel, where the vapor condensates back to fluid travel automatically to the hot region in a cyclic process.

Heat pipes are frequently classified by the structure33. Primary classification of structure can incorporate sintered powder or filaments, porous media, hub grooves, separated annuli, and wrapped screens etc. A few prerequisites for heat channel operation are collected from literature survey and decided to use the Open cell metal porous media. This porous media attached to the evaporator wall. The complete porous media is filled with the fluid which is compatible with the material. Working fluid’s critical point and triple point are properly analyzed and selected Glycerin.Heat pipe control heat by evaporating the fluid uniformly, or to assimilate a high heat flux in the evaporator and reject the heat geometrically31. It is this last component that for the most part makes heat pipe alluring for high heat flux applications.

The geometry of the heat exchanger was built using CATIA V5 R12 and imported to the FLUENT software for analysis of the heat transfer and flow behavior in the heat exchanger.The tetrahedral mesh creation was done utilizing GAMBIT package, Here the space was fit with tetrahedron cells however it also incorporates couple of different sorts of cells in Fig. 3. The meshing is done with a concentration on nodesthe space was split into cells containing 352955 countenances and115976 nodes. The other mesh details are; max cell volume 4.429944e-13, min cell volume 2.248704e-07, max face area 9.696133e-09, min face area 7.421699e-05 and mesh volume 8.193584e-04. The simulationwork is carried out based on the theoretical design of Yang14. The Yang had proposed a following governing equations for fluid and heat flow through the porous media. Energy equation, continuity equation and momentum Equation are given below in Eq-3, Eq-4, Eq-5 respectively.

3

Where Ke = Effective conductivity(based on porous media geometry) F = Inertial Coefficient K = Permeability u = Viscosity of the fluid ƥ = density r = radial coordinate T = Temperature U = Dimension less velocity

Continuity Equation

4Momentum Equation

5The simulation process is initiated at the entrance of the heat exchanger with initial pressure, temperature. In simulation process the k-ԑ model is used for analyzing the mean flow rate and temperature characteristics because this model is capable of describing the both the laminar and turbulent characteristics. The porous media is considered as substrate. Laminar flow is considered during the simulation process and cell condition of the substrate is considered based on inertial and viscous resistance. The simulation is continued until it reached the convergence.

The heat exchanger is analyzed with three different thickness (50mm,60mm,70mm), three different pressures(108041Pa, 147062Pa, 196082Pa) and three different temperatures (60oC, 70oC, 80oC) are maintained and analyzed . These simulated are results are compared with the existing heat exchanger tested with standard experimental setup.For the simulation and experimental work, Taguchi based Design of Experiment was implemented. The method provides a set of nine well-balanced designs which are presented in Table 1. The experimental pressure drop and Temperature on the nine well balanced designs are presented below in table 2.

S.No | Thickness(mm) | Pressure( Pa ) | Temperature(0C) |
---|---|---|---|

D1 | 50 | 108041 | 60 |

D2 | 50 | 147062 | 70 |

D3 | 50 | 196082 | 80 |

D4 | 60 | 108041 | 70 |

D5 | 60 | 147062 | 80 |

D6 | 60 | 196082 | 60 |

D7 | 70 | 108041 | 80 |

D8 | 70 | 147062 | 70 |

D9 | 70 | 196082 | 60 |

Table 2. Experimental Results of Nine well balanced designs

S.No | Pressure P1 (At the inlet) | Pressure P1 (At the Outlet) | Pressure drop ∆P= P1~P2 | Temperature T1(0C) (At the inlet ) | Temperature T2(0C) (At the outlet ) | Temperature difference (0C) |
---|---|---|---|---|---|---|

D1 | 108041 | 106509 | 1532 | 60 | 32 | 28 |

D2 | 147062 | 145564 | 1498 | 70 | 43 | 27 |

D3 | 196082 | 194482 | 1600 | 80 | 48 | 32 |

D4 | 108041 | 106573 | 1468 | 70 | 39 | 31 |

D5 | 147062 | 145530 | 1532 | 80 | 50 | 30 |

D6 | 196082 | 194650 | 1432 | 60 | 27 | 33 |

D7 | 108041 | 106431 | 1610 | 80 | 51 | 29 |

D8 | 147062 | 145628 | 1434 | 70 | 43 | 27 |

D9 | 196082 | 194552 | 1530 | 60 | 36 | 24 |

The simulation results with the temperature and pressure drop in the flow of nine well balanced designs are presented intable 3.

Table 3. Simulation Results of Nine well balanced designs

S.No | Thickness(mm) | Pressure drop (Pa) | Temperature(0C) |
---|---|---|---|

D1 | 50 | 1512 | 17 |

D2 | 50 | 1475 | 25 |

D3 | 50 | 1575 | 33 |

D4 | 60 | 1449 | 24 |

D5 | 60 | 1512 | 37 |

D6 | 60 | 1408 | 20 |

D7 | 70 | 1595 | 34 |

D8 | 70 | 1412 | 27 |

D9 | 70 | 1501 | 19 |

Thecomparative analysisof temperatureand pressure drop is done between theexisting heat exchanger and computationaldata. From the table 2 and 3, we inferred that design no D9 works with better pressure drop and final temperature of the fluid at the outlet. The static pressure variations of design no: D9 are shown in the figure 2. From the results, we can infer that the pressure drop in the newly designed heat exchanger is slightly higher than the existing design due to the copper ridges made on the surface of the heat exchanger. The velocity profile of the fluid in the heat exchanger is shown in the figure 3.

Figure 2. Static pressure vs. curve length

Figure 3. Velocity profile

The existing heat exchangers reduce the temperature on an average of 300C in one pass with pressure drop of 1515 Pa. It is inferred form the result, we can conclude that new modelis capable of reduce an average temperature of450C.The temperature variations over the length of the heat exchanger is shown in figure 4. It is also observed that at low pressure a better reduction in temperature is observed and vice-versa.

Figure 4. Temperature variation Vs curve Length

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