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Drying performance and surface forces of a pair of impinging radial jets

Trocknungswirkung und Oberflächenkraft eines Radialdüsenpaares

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Abstract

The drying performance and the forces generated by a pair of radial jets impinging on a plane surface are studied experimentally and compared with a pair of in-line (axial) jets. The reattachment rings of the radial jets are visualized by liquid-crystal-thermography. The results of the drying experiments are presented in terms of Sherwoodnumbers. The results of the surface force measurements are presented in terms of force coefficients. The effects of flow exit angle, exit velocity, horizontal nozzle spacing and vertical nozzle to surface distance on mass transfer and net surface force are discussed. Results show that drying performance of a pair of radial jets with an exit angle towards the surface of 10° is 18% higher than that of a pair of in-line jets. The surface force of impinging radial jets depends on the exit angle. A global pressure force on the surface is typical for jets directed towards the surface, while a global lifting force is found when the jet is initially directed away from the surface. Radial jets have a high potential in drying applications for force sensitive products and when high drying rates and/or small suction forces are of interest.

Zusammenfassung

Die Trocknungs-und Kraftwirkung von Prallstrahlen auf eine ebene Fläche aus einem Paar von Radialdüsen wird experimentell untersucht und mit einem Axialdüsenpaar verglichen. Die Wiederanglegering der Radialdüsenströmung werden mittels der Flüssigkristallthermographie sichtbar gemacht. Die Ergebnisse der Trocknungsversuche werden mittels Sherwoodzahlen dargestellt. Die Ergebnisse der Strahlkraftmessungen werden durch Kraftbeiwerte dargestellt. Die Einflüsse des Strömungsaustrittswinkels, der Strömungsaustrittsgeschwindigkeit, des horizontalen Düsenabstandes und des vertikalen Abstandes zwischen Düsenmündung und Oberfläche auf den Stoffübergang und auf die Oberflächenkraft werden betrachtet. Die Ergebnisse zeigen, daß die Trocknungsleistung eines Radialdüsenpaares mit einem zur Oberfläche gerichteten Abströmwinkel von 10° um 18% über der des entsprechenden Axialdüsenpaares liegt. Die Oberflächenkraft von radialen Prallstrahlen wird durch den Strömungsaustrittwinkel beeinflußt. Globale Druckkräfte sind typisch für Strahlen, die in Richtung der Oberfläche austreten, während globale Saugkräfte bei Strahlen auftreten, die zunächst von der Oberfläche weggelenkt werden. Radialdüsen haben ein hohes Anwendungspotential bei mechanisch nur gering beanspruchbaren Gütern, wenn hohe Trocknungsraten und/oder geringe Saugkräfte von Interesse sind.

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Abbreviations

A m2 :

area

A E m2 :

nozzle exit cross section

b m:

nozzle exit width

c F :

force coefficient [=F/(ρ L υ 2 E A E )]

c P :

pressure coefficient [=(p−p ∞)/(0.5 ρ L υ 2 E )]

c P J/(kgK):

specific heat at constant pressure

c W J/(kgK):

specific heat of liquid water

D m2/s:

mass diffusivity

d h m:

hydraulic diameter of exit cross section

F N:

net surface force

f :

relative nozzle area [=πr 2 i /s 2]

g m/s2 :

gravitational acceleration

h m:

vertical spacing between RJ nozzle exit and impingement surface

Δh v kJ/kg:

enthalpy of vaporization

L m:

characteristic length

M kg:

mass

M mol kg/kmol:

molecular weight

M kg/s:

mass flow rate

P W N/m2 :

partial pressure of steam

P ∞ N/m2 :

atmospheric or ambient pressure

r i m:

inner nozzle radius

r 0 m:

outer nozzle radius

r R m:

radius of reattachment ring

s m:

centerline spacing between both nozzles

s′ m:

clear spacing between RJ nozzle exits

s 1,s 2 m:

dimensions of plane surface

s nW m:

wall thickness of nozzle (=r 0−r i )

T K:

temperature

t s:

drying time

\(\dot V\) m3/s:

volumetric flow rate

υ E m/s:

average nozzle exit velocity

X ev kg/kg:

relative evaporated mass of water

x kg/kg:

vapor content of the air

β m/s:

average mass transfer coefficient

η Ns/m2 :

absolute viscosity

θ °:

angle of flow exit (RJ nozzle) relative to plane surface

θ R °:

angle of reattachment

v m2/s:

kinematic viscosity

ρ kg/m3 :

density

ρ w kg/m3 :

partial density of steam

τ K:

saturation temperature

φ:

relative humidity

W :

water

F :

air flow

S :

surface

∞:

atmospheric

″:

saturation

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Authors and Affiliations

  1. Lehrstuhl für Wärme- und Stoffübertragung Institut für Thermo-und Fluiddynamik, Ruhr-Universität, D-44780, Bochum

    F. Peper,  W. Leiner &  M. Fiebig

Authors
  1. F. Peper
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  2. W. Leiner
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  3. M. Fiebig
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Peper, F., Leiner, W. & Fiebig, M. Drying performance and surface forces of a pair of impinging radial jets. Heat and Mass Transfer 30, 313–320 (1995). https://doi.org/10.1007/BF01463922

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  • DOI: https://doi.org/10.1007/BF01463922

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