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Spray cooling of extruded EN AW-6082 aluminium alloy sheets: spatial heat transfer coefficients

Abkühlen stranggepresster Platten aus der Aluminiumlegierung EN AW-6082 mittels Spraykühlung – örtliche Wärmeübergangskoeffizienten

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Abstract

Quenching profiles during their extrusion with subsequent ageing can increase their strength by a factor of 1.5–2 compared to the as-extruded state. Water-air spray cooling is a quenching technology which allows the desired profile temperature to be reached with high accuracy and which is characterised by low energy demand. To apply this cooling technique correctly, the heat transfer coefficient’s dependence on the surface temperature and properties of the extruded profile, made from EN AW-6082 alloy, and on the spray cooling parameters was determined using the lumped heat capacitance method. The absence of a significant impact of the spraying distance and inclination angle on the average heat transfer coefficient within the investigated range was noted. Dependencies of the relative heat transfer coefficient on the temperature and the distance to the spray axis were derived. It was shown that these dependencies, which can be used for modelling such processes, are practically identical for all considered spraying parameters.

Zusammenfassung

Das Abschrecken von Profilen beim Strangpressen mit anschließendem Auslagern kann im Vergleich zum stranggepressten Zustand zu einer Festigkeitssteigerung um den Faktor 1,5 bis 2 führen. Eine der einsetzbaren Abschrecktechnologien ist die Wasser-Luft-Spraykühlung, die eine gezielte Einstellung der Profiltemperatur erlaubt und durch einen geringen Energieverbrauch charakterisiert ist. Zur Anwendung dieser Abkühltechnologie wurde an stranggepressten Profilen der Aluminiumlegierung EN AW-6082 die Abhängigkeit des Wärmeübergangskoeffizienten von der Oberflächentemperatur, den entsprechenden Profileigenschaften und den Sprayparametern mit der Methode der konzentrierten Wärmekapazitäten ermittelt. Ein signifikanter Einfluss des Sprühabstands und –winkels auf die gemittelten Wärmeübergangskoeffizienten im untersuchten Bereich wurde nicht festgestellt. Abhängigkeiten des relativen Wärmeübergangskoeffizienten von der Temperatur und dem Abstand zur Sprühachse wurden hergeleitet. Für die hergeleiteten Abhängigkeiten, die für eine Modellierung solcher Prozesse herangezogen werden können, wurde gezeigt, dass diese für alle untersuchten Sprühparameter anwendbar sind.

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Abbreviations

A :

cooling surface area (mm2)

Bi :

Biot number (–)

c p :

specific heat (J/(kg·K))

L :

distance from the nozzle exit to the plate surface (mm)

p A :

air pressure (MPa)

p W :

water pressure (MPa)

q :

water consumption (g/s)

r :

radial component (mm)

R :

specimen radius (mm)

s :

plate thickness (mm)

t :

time (s)

T :

surface temperature (°C)

T 0 :

initial temperature (°C)

T ∞ :

temperature of the cooling medium (°C)

V :

volume (mm3)

α :

heat transfer coefficient (W/(m2·K))

α A :

temperature-dependent averaged heat transfer coefficient per impinged area (W/(m2·K))

α maxA :

maximum temperature-dependent averaged heat transfer coefficient per impinged area (W/(m2·K))

α T :

spatial-dependent heat transfer coefficient (W/(m2·K))

\(\bar{\alpha }\) :

mean heat transfer coefficient averaged over both the total specified impingement area and the surface temperature range (W/(m2·K))

β :

jet aperture angle (°)

γ :

angle between the nozzle and perpendicular to the plate (°)

ρ :

density (kg/m3)

λ :

thermal conductivity (J/(K·m·s))

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Authors and Affiliations

  1. Department of Metal Forming, National Metallurgical Academy of Ukraine, Dnipropetrovsk, Ukraine

    O. Golovko & I. Frolov

  2. Institut für Werkstoffkunde (Materials Science), Leibniz Universität Hannover, Garbsen, Germany

    D. Rodman & F. Nürnberger

  3. Lehrstuhl für Werkstoffkunde (Materials Science), University of Paderborn, Paderborn, Germany

    O. Grydin & M. Schaper

Authors
  1. O. Golovko
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  2. I. Frolov
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  3. D. Rodman
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  4. F. Nürnberger
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  5. O. Grydin
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  6. M. Schaper
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Correspondence to D. Rodman.

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Golovko, O., Frolov, I., Rodman, D. et al. Spray cooling of extruded EN AW-6082 aluminium alloy sheets: spatial heat transfer coefficients. Forsch Ingenieurwes 78, 131–137 (2014). https://doi.org/10.1007/s10010-014-0181-y

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