Corona discharge effects on heat transfer and pressure drop in tube flows

This work presents and discusses the results of a series of experiments inv estigating effects from corona discharge in air on the heat transfer rate a nd on the pressure drop in tube flows. Two electrode geometries were studie d: a single wire electrode, concentric with the grounded tube wall and dual equipotential wire electrodes which were offset 0.4 cm from center in the horizontal plane. Both positive and negative discharge were examined for th e single-wire geometry, at Reynolds numbers in the range 1,000 less than or equal to Re-D less than or equal to 20, 000. The dual wire geometry was st udied using positive polarity discharge only, over the range Re-D = 1,000 t o Re-D = 10, 000. Heat transfer rates were determined at electrode potentia ls from 6.00 kV (DC) to 7.75 kV (DC), depending on polarity and electrode c onfiguration. Baseline data were also obtained with the electrode(s) at gro und potential. Results demonstrate increases in the Nusselt number of more than two hundre d per cent over the values obtained in the absence of discharge. Relative i ncreases in the friction coefficients were generally comparable to the corr esponding Nusselt number enhancement. The extent of the increase in either quantity was highly dependent on discharge current and on the Reynolds numb er. The relative enhancements of both Nusselt number and friction loss coef ficient were generally reduced at higher Reynolds numbers (Re-D greater tha n or equal to 5000). However, the fall-off of enhancement with Reynolds num ber was less pronounced in the offset, dual-electrode geometry. Results suggest the enhancement mechanism may significantly depend on the e lectrode geometry, independent of the geometry effects on discharge current . The observed trends are discussed in the context of current theory.