Performance Investigation of Multiple-Tube Ground Heat Exchangers for Ground-Source Heat Pump
Issue:
Volume 2, Issue 5, September 2014
Pages:
103-107
Received:
13 September 2014
Accepted:
30 September 2014
Published:
10 October 2014
Abstract: The present study aims to investigate the performance of multiple-tube ground heat exchangers (GHEs). The multiple-tube GHEs with a number of pipes installed inside the borehole were simulated. Thermal interferences between the pipes and performance of multiple-tube GHEs are discussed. Increasing the number of inlet tube in the borehole increases the contact surface area and then leads to increase of heat exchange with the ground. However, ineffective of heat exchange in the outlet tube caused by thermal interferences from the inlet tube reduces the heat exchange rate for the GHEs. The GHE performances increase of 9.1 % for three-tube, of 13.6 % for four-tube, and of 20.1 % for multi-tube compared with that of the U-tube. The four-tube and multi-tube GHEs which consist of four pipes as heat exchange pipes where the multi-tube GHE provides better performance than that of the four-tube GHE. This fact indicates that thermal interferences between the pipes affect the performance. Thermal interferences between the pipes should be considered.
Abstract: The present study aims to investigate the performance of multiple-tube ground heat exchangers (GHEs). The multiple-tube GHEs with a number of pipes installed inside the borehole were simulated. Thermal interferences between the pipes and performance of multiple-tube GHEs are discussed. Increasing the number of inlet tube in the borehole increases t...
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Single and Multicomponent Droplet Models for Spray Applications
Shah Shahood Alam,
Ahtisham Ahmad Nizami,
Tariq Aziz
Issue:
Volume 2, Issue 5, September 2014
Pages:
108-126
Received:
16 October 2014
Accepted:
24 October 2014
Published:
10 November 2014
Abstract: An unsteady, spherically symmetric, single component, diffusion controlled gas phase droplet combustion model was developed first, by solving numerically the time dependent equations of energy and species. Results indicated that flame to droplet diameter ratio (flame standoff ratio) increased throughout the droplet burning period, its value being much smaller than that of the quasi-steady case, where it assumes a large constant value. Effects of fuels on important combustion characteristics suggested that combustion parameters were influenced primarily by the fuel boiling point. Droplet mass burning rate variation was smallest for ethanol in comparison with methyl linoleate (biodiesel) and n-heptane. Also, effects of fuels on CO, NO, CO2, and H2O concentrations were determined from the point of view of getting a qualitative trend.For multicomponent spherical combustion of a heptane-dodecane droplet, it was observed that the mass fraction of heptane decreased abruptly to a minimum value as the droplet surface was approached. For a 200μm hexane-decane droplet (at its boiling point), vaporising in conditions of 1 atm and 1000K with Le_l = 10, it was observed that mixing of air and fuel vapour resulted in a higher concentration of hexane at the droplet surface at the end of droplet lifetime thereby altering the vaporisation behaviour. Other conditions remaining same, an increase in Lewis number resulted in a higher mass fraction of hexane being present at the droplet surface. A detailed multicomponent (MC) droplet vaporisation model (diffusion limit model with convection and no internal liquid circulation) was also evolved by numerically solving the transient-diffusive equations of species and energy for a 280 μm (heptane-dodecane) droplet vaporising at 1 atm and 1000 K with Re_σ=100, and Le_l= 10. The present MC model was compared with other existing models and was found to be simpler and quite accurate. The submodels developed in the present work can be implemented in spray analysis.
Abstract: An unsteady, spherically symmetric, single component, diffusion controlled gas phase droplet combustion model was developed first, by solving numerically the time dependent equations of energy and species. Results indicated that flame to droplet diameter ratio (flame standoff ratio) increased throughout the droplet burning period, its value being m...
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