Heat transfer during pulsating liquid jet impingement onto a vertical wall
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ORIGINAL
Heat transfer during pulsating liquid jet impingement onto a vertical wall J. Wassenberg 1
&
P. Stephan 1 & T. Gambaryan-Roisman 1
Received: 2 July 2020 / Accepted: 21 September 2020 # The Author(s) 2020
Abstract Liquid jet impingement is used for cooling and cleaning in various industrial branches. The advantages of jet impingement include high heat and mass transport rates in the vicinity of the impingement point. Pulsating liquid jets impinging on horizontal substrates with a pulsation frequency around 100 Hz have been shown to increase the cooling efficiency in comparison to jets with continuous mass flow rates. The influence of jet pulsation on cooling efficiency for impingement of horizontal jets onto vertical walls has not yet been investigated. In the case of a vertical heated wall, gravity contributes to the liquid flow pattern. In particular, if the time span between two pulses is sufficiently long, the liquid drainage from the region above the impingement point can contribute to heat transport without increasing the average flow rate of the cooling medium. In this work, the influence of pulsations on heat transfer during impingement of a horizontal liquid jet onto a vertical wall is investigated experimentally for the pulsation frequency range 1–5 Hz. The results regarding increase of heat transfer efficiency are related to flow patterns developing by impingement of successive pulses, as well as to the liquid splattering. Nomenclature AH c dN DC
f fr h L ˙ : ˙ : ˙ : M ; M r; M s
Q˙ : R r
rs Surface of cylindrical heater, m2 Specific heat capacity, J kg−1K−1 Nozzle diameter (orifice), m Duty cycle of pulsation (%), ratio of durations of nozzle operating time and one period, DC = τon/τ = τon f Pulsation frequency, Hz Frame rate (high speed camera), Hz Heat transfer coefficient, W m2 K−1 Nozzle distance, distance between nozzle and target, m Mass flow through nozzle, remaining mass flow, splattered mass flow, kg s-1 Total heat flow introduced, W Radius of cylindrical heater, m Radial coordinate measured from point of impingement, m
Tw,Tl t, ts ujet X, Y
μ ν ξ ρ τ, τon, τoff Re Sr
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00231-020-02973-z) contains supplementary material, which is available to authorized users. * J. Wassenberg [email protected] 1
Institute for Technical Thermodynamics, Technische Universität Darmstadt, Darmstadt, Germany
St Stφ St
Radial position of splattering, at which droplets leave the wall flow, rs = 5.71dN Tw, Tl Measured wall temperature, liquid temperature (water) at nozzle inlet, K time, shutter time (high speed camera), s Free liquid jet velocity m s−1 Extent of the radial flow zone in the horizontal and vertical direction measured from the point of impingement m Dynamic viscosity (water), kg m−1s−1 Kinematic viscosity (water), m2s−1 Splattered mass fraction, ˙ : ˙ : ˙ : ˙ : ξ ¼M s =M ¼ 1−M r =M Density (water), kg m−3 Duration of one period, nozzle operating time, nozzle pause time, s Reynold
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