Impingement heat/mass transfer to hybrid synthetic jets and other reversible pulsating jetsby Z. Trávníček, T. Vít

International Journal of Heat and Mass Transfer

About

Year
2015
DOI
10.1016/j.ijheatmasstransfer.2015.01.125
Subject
Condensed Matter Physics / Mechanical Engineering / Fluid Flow and Transfer Processes

Text

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Article history:

Received 3 October 2014

Received in revised form 25 January 2015

Accepted 25 January 2015

Keywords:

Impinging jet

This study focused on round, hybrid synthetic (non-zero-net-mass-flux) jets impinging on a wall. To come.g. [3–7]. The majority of previous studies focused on continuous (steady-flow) IJs. Despite the ability of steady IJs to achieve very high heat fluxes, further intensification of the transport processes seems possible through an incorporation of unsteadiness effects.

However, pulsations do not automatically lead to enhanced rates of heat transfer; the effect can sometimes be found to increase at its use resulted ransfer at a rather 12] used a

J with a p flapping motion, which, based on their conclusions, result 40–70% greater heat transfer rate than that from a continu

Liu and Sullivan [13] investigated the heat transfer to round IJs at relatively small nozzle-to-wall spacings. They obtained either an enhancement or reduction of the local heat transfer by controlling the development of the vortex structure by forcing the excitation frequency to be near the natural frequency of the unexcited free jet or its subharmonic frequency, respectively.

Gau et al. [14] concluded that excitation at the natural frequencies of the unexcited free jet (at the first or second subharmonic ⇑ Corresponding author.

E-mail addresses: tr@it.cas.cz (Z. Trávnícˇek), tomas.vit@tul.cz (T. Vít).

International Journal of Heat and Mass Transfer 85 (2015) 473–487

Contents lists availab

International Journal of H .ehas resulted in significant research. The most important early results were collected in monographs, such as the outstanding book by Dyban and Mazur [1] and the distinguished work by Martin [2]. Since then, several comprehensive reviews have appeared, to generate a self-oscillating IJ, they concluded th in a 45% enhancement of the impingement heat t high orifice velocity of 153 m/s. Camci and Herr [ oscillating nozzle to generate a self-oscillating Ihttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.125 0017-9310/ 2015 Elsevier Ltd. All rights reserved.fluidic eriodic ed in a ous IJ. excited1. Introduction

Submerged impinging jets (IJs) and impingement heat and/or mass transfer on exposed walls have been widely studied in the past. Because of their high heat and mass transfer values in single-phase flows, there are large number of applications for IJs. This the transfer, sometimes to decrease it, or to have a negligible effect – see, e.g. Herman [8], Herwig et al. [9], and Persoons et al. [10].

There are many ways to generate a pulsatile character of IJs, which utilize mechanical vibration/motion, alternate blowing, and fluidic oscillators with no moving parts. For example, when

Page et al. [11] used a straight circular pipe nozzle with a collarReversible pulsating jet

Synthetic jet

Mass transfer

Heat transfer

Naphthaleneplete this study, two additional variants of reversible pulsating jets were investigated, namely synthetic (zero-net-mass-flux) jets and mixed pulsed jets (pulsed jets containing an additional blowing component). For comparison purposes, the continuous jet was used. The working fluid was air. The Reynolds numbers ranged from 4000 to 6000, the dimensionless stroke lengths were 14–16, and the dimensionless orifice-to-wall distances were 2–16 (both related to the orifice exit diameter of 8 mm). The experiments used flow visualization, single-sensor hot-wire measurements, and mass transfer measurements on the wall using the naphthalene sublimation technique. The local heat transfer coefficient, expressed as a

Nusselt number, was evaluated using the heat/mass transfer analogy.

All tested jets exhibited relatively flat, nearly top-hat velocity profiles. An increase of the Reynolds number by an additional blowing component resulted in a heat/mass transfer enhancement. The flow oscillations (for the present geometry and driven parameters) caused a heat/mass transfer enhancement of 12–40%.

The main outcome was that the 26% larger flow rate of the hybrid synthetic jet, versus the conventional synthetic jet, resulted in an 18% increase in the heat transfer rate. These gains were caused by a partial rectification effect of incorporated fluidic diodes, without consuming any additional energy and without introducing movable parts.  2015 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b s t r a c tImpingement heat/mass transfer to hybr and other reversible pulsating jets

Z. Trávnícˇek a,⇑, T. Vít b a Institute of Thermomechanics v.v.i, Czech Academy of Sciences, Dolejškova 5, 182 00 P b Technical University of Liberec, Studentská 2, 461 17 Liberec, Czech Republic journal homepage: wwwsynthetic jets e 8, Czech Republic le at ScienceDirect eat and Mass Transfer l sevier .com/locate / i jhmt

Nomenclature

CJ continuous (steady) jet

D orifice exit diameter, see Fig. 1

Dn mass diffusion coefficient of naphthalene vapor in air f actuating frequency h local heat transfer coefficient hm local mass transfer coefficient

H orifice-to-wall distance, see Fig. 1

HSJ hybrid synthetic (non-zero-net-mass-flux) jet

IJ impinging jet k thermal conductivity of air

L0 and L0A extruded fluid column length, defined as U0 T and U0A

T, respectively

MPJ mixed pulsed jet

Nu Nusselt number, hD/k r radial coordinate, see Fig. 1

Pr Prandtl number

ReSJ, ReHSJ, and ReMPJ Reynolds numbers of SJ, HSJ, and MPJ, respectively; U0A D/m

ReCJ Reynolds number of the continuous jet, UCJD/m

Sc Schmidt number

Sh Sherwood number, hm D/Dn

SJ synthetic (zero-net-mass-flux) jet t time

T time period, 1/f

TE extrusion stroke duration u0 orifice velocity

U velocity averaged over time

UA average orifice velocity defined by the integration over the entire cycle, see Eq. (3)

UCJ time- and spatial-averaged orifice velocity of the continuous jet

Uf periodic component of the velocity related to the actuating frequency f

U0 orifice velocity averaged over time, see Eq. (1)

U0A time- and spatial-averaged orifice velocity, see Eq. (2) u velocity u0 RMS value of fluctuating velocity component