Rapid cooling by direct expansion of coolant through an orifice

This article describes an experimental study of a novel rapid cooling scheme for sensor optics. The scheme involves direct expansion of a coolant from a high pressure to ambient conditions, resulting in a two-phase mixture with a high cooling capacity. Such a system is reliable, lightweight, and requires no servicing. Therefore it can provide significant weight, volume, and/or operational advantages over other expendable cooling schemes such as stored cryogens and conventional Joule-Thompson coolers. Results are presented which demonstrate the feasibility of this scheme for cooling the optics to temperatures of 180 K or less in less than 75 s. Various coolants were considered and the relative merits of these for rapid cooling are discussed. Nomenclature A = orifice flow area g = gravitational acceleration hfK = latent heat of vaporization hm[n = film boiling heat transfer coefficient k = vapor thermal conductivity m = mass flow rate Ph = downstream pressure P() = upstream pressure T = temperature xe = quality after expansion Armin = Leidenfrost temperature difference Ap = liquid-vapor density difference fji = kinematic viscosity v = dynamic viscosity pv - vapor density p, = liquid density a = surface tension coefficient