(227h) Thermohydraulic Characteristics of TIO2-Water Nanofluids in Dimpled Tubes Fitted with Twisted-Tapes

Characteristics of heat transfer enhancement, friction factor and thermal performance in dimpled tubes equipped with twisted tape have been examined and carried out. In the present investigation, two enhancement devices are applied for increasing the heat transfer rate in turbular heat exchanger. One is the dimpled tubes used as turbulence flow generators while twisted tape used as swirl flow generators. Three twisted-tapes of different twist ratios, y/W = 2.0, 3.0, and 4.0, were introduced in each run which inserted in the heat transfer test section that heated electrically imposing axially and circumferentially constant wall heat flux (UHF) boundary conditions.  TiO₂-water as the working fluid is passed dimpled tubes in a Reynolds number between 5000 and 15,000.  Nanofluids consisting of TiO₂-water at TiO₂ concentrations between 0.1% and 0.3% by volume, were utilized as working fluids in the dimpled tubes equipped with twisted-tape.  The experimental results are displayed in terms of Nusselt number (Nu) and friction factor (f) as a function of Reynolds number (Re).  At similar operating conditions, the dimpled tubes equipped with twisted-tape consistently provided superior Nusselt number, friction factor and thermal performance factor to those of dimpled tube alone.  The use of y/W = 2.0 leads to higher Nusselt number values than that of y/W = 3.0, and 4.0.   For all the devices used, the thermal performance tends to decrease with the rise of Reynolds number and to be nearly uniform for Reynolds number over 16000.  It is interesting that the thermal performance factors were apparently above unity this indicates the beneficial effect for the energy saving by the uses of the combined techniques. The experimental also results revealed that the heat transfer rate increased with increasing nanofluid concentration.  In addition, correlations for Nusselt number, friction factor and performance evaluation criteria to assess the real benefits in using dimpled tubes fitted with twisted-tape at different TiO₂ concentrations are performed.