Heat transfer in a pipe under conditions of transient turbulent flow

This paper is concerned with the response of fluid temperature within a hea ted pipe to imposed excursions of flow rate. Experiments are reported in wh ich measurements of wall temperature and local fluid temperature were made with fully developed turbulent flow of water in a uniformly heated tube dur ing and after ramp-up excursions of flow rate between steady initial and fi nal values. The fluid temperature measurements were made using a traversabl e temperature probe incorporating a thermocouple capable of responding to t urbulent fluctuations of temperature. Local values of mean temperature and RMS temperature fluctuation were obtained by ensemble averaging the results from many tests in which the same flow excursion was applied in a very rep eatable manner with fixed values of inlet fluid temperature and heat flux. Further measurements were made under conditions of steady flow rate at a nu mber of values over the range covered in the transient flow experiments. Th e results obtained in the experiments with transient flow show that there i s a significant delay in the variation of ensemble-averaged wall temperatur e and striking perturbations in the variations of RMS fluctuation of wall t emperature and local fluid temperature. These stem from the delayed respons e of turbulence to the imposed excursions of flow rate. They provide indepe ndent confirmation of ideas concerning the modelling of time scales for the production and diffusion of turbulence in pipe flow which were developed b y the present authors in the course of earlier work. Ensemble-averaged loca l fluid temperature also varies in an unusual manner. Instead of falling mo notonically with increase of flow rate, as might be expected, it starts to rise at some stage, reaches a peak value and then falls again. The release of heat stored in the pipe wall contributes to this behaviour. Computationa l simulations of the present experiments were performed using a spatially f ully developed formulation of the equations for unsteady turbulent flow and heat transfer in a boundary layer utilising turbulence models of low Reyno lds number, k-epsilon type. Comparisons between predicted and measured vari ations of temperature are presented in the paper. These show that the predi ctions differ significantly from model to model and that detailed agreement with experiment is not obtained using any of the models. However, certain interesting features of the observed temperature variations, such as a dela y in the response of outer wall temperature and perturbations in local flui d temperature, are present in the computed results. (C) 1999 Elsevier Scien ce Inc. All rights reserved.