Showing 5 results for Thermal Conductivity Coefficient
Milad Azarfar, Hamid Niazmand, Hoda Talebian,
Volume 14, Issue 8 (11-2014)
Abstract
During the past few decades, growing global concern about environmental problems, caused by widespread use of fossil fuels, attracts more research attention toward adsorption systems technology. However, one of the main problems of these systems is the poor heat transfer rate in adsorbent bed due to its low thermal conductivity. In the present study, extended surfaces and metal piece additives are applied to the adsorbent bed in order to numerically investigate the effect of heat transfer enhancement on the adsorption system performance. Employing metal pieces increases effective thermal conductivity of the bed by at least 100%. Results indicate that decreasing fin space and fin height and adding metal pieces to the adsorbent bed reduce the cycle time which finally improves the system specific cooling power. However, it is worth mentioning that the effect of metal piece additives on the cycle time reduction and specific cooling power improvement decreases at smaller fin spaces. Moreover, results show that the increase of fin height improves the coefficient of performance while decreases the specific cooling power of the system. On the contrary, the reduction of fin space simultaneously increases the coefficient of performance and the specific cooling power of the adsorption system.
Sadegh Sadeghzadeh, Navid Rezapour,
Volume 16, Issue 1 (3-2016)
Abstract
In this paper, efficiency of defected graphene nano ribbon incorporating with additional nanoparticles on mass detection operations is studied via the Reverse Non Equilibrium Molecular Dynamics (RNEMD) method. Thermal conductivity management of this structure is challenging because of imposed losses in electrical conductivity and any procedure could manage the thermal conductivity of graphene will be useful. In this paper it is observed that on the mass detection operation, due to the porosity generation in the nano ribbon surface or even creation of external nanoparticles, thermal properties of graphene change considerably. This should be noted in calibration of graphene based mass sensors. In summary, results show that the graphene’s thermal conductivity would reduce by increasing the concentration of nanoparticles and thermal conductivity of graphene is higher when porosities and impurities are at the edges. This indicates that the location of vacancies and nanoparticles influences the thermal conductivity. For a better thermal management with the help of nanoparticles, wither respect to the porosities, addition of nanoparticles decrease the thermal conductivity more and more. By increasing the cavity’s diameter from 0.5nm to 4.4nm in a specific single layer graphene, thermal conductivity was reduced from 67 W/mk to 1.43 W/mk.
M. Khatibi , M. Mohammadzadeh Kowsari, H Niazmand,
Volume 19, Issue 3 (3-2019)
Abstract
In this study, the thermo-physical properties effects of the heat exchanger body on the adsorption chillers performance have been investigated. For this purpose, an adsorbent bed with a rectangular finned flat-tube heat exchanger is simulated by employing a three-dimensional control volume scheme. Furthermore, silica gel SWS-1L-water has been used as a working pair. In order to investigate the effects of thermo-physical properties of the heat exchanger body material, two main parameters including the thermal conductivity coefficient and the volumetric thermal capacity are examined. Also, the effects of these parameters along with variations of the fin height and fin pitch on the specific cooling power (SCP) and the system coefficient of performance (COP) are investigated. The results indicated that the SCP increases with the increase in thermal conductivity coefficient up to a certain value, which increases and decreases with the increase in fin height and fin pitch, respectively. The results also showed that the effects of the volumetric thermal capacity on the SCP are negligible such that it can be considered independent of the heat exchanger body material volumetric thermal capacity. Unlike the SCP, the COP is strongly influenced by the volumetric thermal capacity. The increase in volumetric thermal capacity results in decreasing the COP. The slope of the decrease in the COP decreases with increasing the fin height and pitch. Also, by increasing the thermal conductivity coefficient, the COP slightly decreases.
H. Ghaderi , A. Ghasemi , S. Rouhi , E. Mahdavi ,
Volume 19, Issue 9 (9-2019)
Abstract
In this paper, the thermal conductivity coefficient of multi-walled boron nitride nanotubes has been investigated, using molecular dynamics simulation based on the Tersoff and Lenard Jones potential functions. The effects of diameter, length, and temperature on the thermal conductivity of double-walled boron nitride nanotubes have been studied. Also, by considering the 2, 3, 4, and 5-wall nanotubes, the effect of number of walls on the thermal conductivity of boron nitride nanotubes were studied. Finally, by considering of zigzag and armchair nanotubes, the effect of chirality has been investigated. The results showed that the thermal conductivity coefficient of double-walled boron nitride nanotubes increases by increasing the diameter of nanotubes and decreases by increasing temperature. It had been demonstrated that with 73% and 82% increase in the outer diameter of nanotubes, the thermal conductivity increases 93% and 98%, respectively. Furthermore, regarding to the chirality, the armchair nanotubes have a higher thermal conductivity than the zigzag ones. Also, the simulation results showed that thermal conductivity coefficient increases by increasing the length of boron nitride nanotubes and 50% increase of effective nanotube length increases the thermal conductivity by 25% approximately. Finally, by studying the effect of the number of walls, it is concluded that in the same length and temperature, nanotubes with higher number of walls have higher thermal conductivity coefficient in comparison.
F. Aminifar, A. Ghafouri, A. Falavand Jozaei,
Volume 20, Issue 10 (10-2020)
Abstract
Heat exchangers facilitate the transfer of heat between fluids with different temperatures. Compared with solids, most fluids have lower heat transfer coefficients and as a result, the use of high heat transfer coefficient solid particles as additives can increase the convective heat transfer coefficient of the fluid. In this study, the effect of the addition of nanoparticles to the base fluid (deionized water), application of triangular-cut twisted tapes as well as corrugation of shell and tube type heat exchangers pipes, is investigated on heat transfer values, friction coefficient variations as well as variations in performance evaluation criterion. The effects of addition of 0.7 and 1% magnesium-oxide nanoparticles on heat transfer coefficient improvements is investigated and the results of simultaneous application of magnesium-oxide water nanoparticles, corrugated pipes, and twisted tapes are compared. Comparisons against the basic conditions (deionized water without nanofluid, corrugated pipes or triangular-cut twisted tapes) indicate a 48% increase in thermal performance, a minuscule increase of 6.3% in friction coefficient and a 46% increase in the performance evaluation criterion as a result of the application of %0.7 magnesium-oxide water nanoparticles, use of corrugated pipes and triangular cut twisted tapes on the inner surface of shell and tube heat exchanger piping. Also, the application of 1% magnesium-oxide water nanofluid, and simultaneous use of corrugated pipes and triangular-cut twisted tapes on shell and tube heat exchanger piping inner surface results in a 72% increase in thermal performance, a minuscule increase of 6.9% in friction coefficient and a 70% increase in the performance evaluation criterion.
