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Transient heat transfer is investigated in forced flow of He II for velocities up to 22 m/s. The flow is generated in a 10 mm ID, 0.85 m long straight test section instrumented with heaters, thermometers and pressure transducers. Rectangular heat pulses are generated in the flow and the temperature is measured at several locations as the pulses are carried by the forced flow with their shape being transformed by counterflow heat transfer. The linear pressure drop in the flow also results in a linear temperature gradient due to the Joule-Thomson effect and therefore has an influence in the heat transfer process. The effectiveness of the counterflow heat transfer also appears to decrease at the highest flow velocities. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]