Heat transfer at multiple-intermittent impacts of a hollow cone spray q A.L.N. Moreira * , M.R.O. Pana ˜o Instituto Superior Te ´cnico, Mechanical Engineering Department, Av. Rovisco Pais 1049-001 Lisboa, Portugal Received 3 November 2005 Available online 27 June 2006 Abstract The present work is aimed at quantifying the eﬀects of frequency in the heat transfer at multiple-intermittent impacts of a hollow cone spray in a way to contribute to the development of advanced heat transfer techniques. The ﬂow conﬁguration is that of a spray impacting perpendicularly onto an aluminium ﬂat plate located at 55 mm. The experiments are conducted at prescribed temperatures ranging from local nucleate/boiling to local transition regimes of heat transfer and with frequencies of injection from 10 Hz to 30 Hz with durations of 5 ms. Analysis is based on spatial resolved measurements of the instantaneous surface temperature during the period of injection, pro- cessed in order to obtain estimates of the instantaneous heat ﬂux. The results show that the local heat transfer is enhanced when the frequency of injection increases from 10 Hz to 20 Hz, but deteri- orates with a further increase of frequency, as a clear indication of the interaction between successive intermittent injections. However, the total heat ﬂux removed over the entire area of impact follows the behaviour of typical boiling curves with an Overall Nukiyama Temperature. In the overall nucleate boiling regime, the total heat transfer is dominated by a thin ﬁlm boiling mechanism which lead to breakdown of the liquid at a nearly constant surface temperature, regardless of frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid ﬁlm. A correlation is found for the total heat ﬂux removed from the surface within the nucleate regime which accounts for the combined non-linear eﬀects of surface temperature and pulse frequency, h _ q 00 i _ m f ðh fg Þ b  A 1 impact ¼ 0:17  T w T f ðDt inj  f inj Þ 0:284ðDt inj  f inj Þþ 0:046  T w T f 1  0:112 T w T f    0:077: The critical heat ﬂux is shown to be well predicted by correlations reported in the literature for continuous water and dielectric sprays. However, the present multiple-intermittent spray allows achieve higher speciﬁc heat ﬂuxes with higher eﬃciencies. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Spray cooling; Pulsed spray; Heat transfer; Overall boiling curve 1. Introduction Cooling at high heat ﬂuxes is currently a barrier to devel- oping a number of high density power and electronic sys- tems such as those used in space vehicles and satellites, or thermo-photovoltaic systems. Vaporization of a liquid is the only way to achieve such large amounts of heat at low temperatures from small areas and has been considered in most cooling technologies, e.g., Mudawar [1]. In this con- text, spray cooling allows removing much higher heat ﬂuxes than saturated pool boiling, despite the residence time of the liquid on the surface may be smaller than in a pool. Each individual droplet splash, rebound, spread or stick [2], 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.04.004 q Paper submitted to the International Journal of Heat and Mass Transfer, revised in March 2005. * Corresponding author. Tel.: +351 21 841 7875; fax: +351 21 849 6156. E-mail address: moreira@dem.ist.utl.pt (A.L.N. Moreira). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 4132–4151