Energy and Buildings 43 (2011) 2656–2661 Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild Reducing temperature stratification using heated air recirculation for thermal energy saving Mostafa Rahimi ∗ , Kaveh Tajbakhsh Department of Mechanical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran article info Article history: Received 13 January 2011 Received in revised form 28 May 2011 Accepted 17 June 2011 Keywords: Thermal energy saving Circulating heated air abstract Using discrete heat sources such as a radiator or a gas heater, a large amount of heated air is accumulated under the ceiling surface where it is usually far enough from the occupancy zone. The idea of transferring this heated air into the occupancy zone has been the subject of the present study. A full length-scale model of a compartment was constructed and equipped with a radiator and an air circulating mechanism. Temperature distribution on the symmetry plane was specified for the two cases of usual heating and heating with air circulation. Heat loss from the surfaces of the enclosure was also determined using thermal maps of both the internal and the external surfaces of the enclosure. In order to examine the effect of circulating air on the thermal energy saving quantitatively, a numerical model was developed and validated using experimental results. The input heat rate of the enclosure was compared for the two cases of usual heating and heating with air circulation considering different heights for a typical heating space. Based on the results, having the same mean air temperature within the occupancy zone, the input heat rate of a compartment could be moderated using circulating air criterion. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Various types of heaters are utilized in the heating system of buildings. They could be classified considering the extent of the surface from which heat is transferred into the compartment to be heated. In some applications, an extended surface like a heating panel is installed on a vertical or horizontal surface of a compart- ment. Using discrete heat sources such as radiators or gas heaters is another alternative. Heat is transferred by coupled mechanism of convection and radiation from the heated surfaces into the sur- roundings and a temperature distribution is finally established within the environment under steady heat flow condition. The resulted temperature profile is somewhat undesirable when using discrete heating units. Using this type of heating system, warm air goes up and accumulates beneath the ceiling surface so that the air temperature will have a positive gradient in the upward vertical direction. As a result and depending on the geometrical and phys- ical specifications of the compartment, more thermal energy has to be supplied to the compartment to meet the thermal comfort requirements. Heat transfer mechanism from different types of heaters and the resultant temperature distribution within a compartment have ∗ Corresponding author at: Department of Mechanical Engineering, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran. Tel.: +98 451 5512910; fax: +98 451 5512904. E-mail addresses: rahimi@uma.ac.ir, mosrah12@yahoo.com (M. Rahimi). been reported in a large number of published studies. Abdul-Jabbar [1] presented an extensive review of the studies regarding convec- tive heat transfer over the vertical and horizontal isolated surfaces applicable for building geometries. Karadag et al. [2] studied the effect of room dimensions on the floor average Nusselt number. Based on their findings, average Nusselt number over the floor of a room increases when the room dimensions as well as the temperature difference between the ceiling and the indoor air is increased. Ergin [3] investigated the surface radiation coupled with natural convection and conduction for a two-floor enclosure exper- imentally. Radiation between the surfaces within the enclosure including the heater surfaces was computed using the net-radiation method in that study. Using a numerical investigation, Park and Holland [4] pointed out that the flow and temperature fields are affected by the location of a convective heat source in thermal dis- placement ventilation. Based on their findings, as the heat source rises the convective heat gain in an occupied zone becomes less significant. Ho et al. [5] analyzed the effect of a ceiling fan on the thermal comfort characteristics numerically. They found that these characteristics shift considerably toward the cooler scale as the normal air speed from the fan is increased, so that higher heat load would be permissible in the room while maintaining the same comfort level. The objective of the present study has been to reduce the strat- ification of the indoor air when using discrete heater units, so that the input heat energy could be moderated. In order to verify the temperature gradient in the vertical direction, the temperature dis- tribution over the central plane of a cavity measured under steady 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.06.025