Effect of inlet geometry and heating on the fully developed friction factor in the transition region of a horizontal tube

International Journal of Heat and Mass Transfer

Garach

2014

An experimental investigation of heat transfer and pressure drop in rectangular micro-channels was conducted for water in the laminar and transitional regimes for three different inlet configurations.The inlet types under consideration were the sudden contraction, bellmouth, and swirl inlet types, and hydraulic diameters of 0.57 mm, 0.85 mm, and 1.05 mm were covered. It was found that the critical Reynolds number and the transitional behaviour in terms of heat transfer coefficients and friction factors were influences significantly by the inlet type. For the sudden contraction inlet type, which were investigated for both adiabatic, as well as diabatic cases, adiabatic friction factors were predicted well by the laminar Shah and London correlation, while diabatic friction factors were decreased with an increase in wall heat flux. The sudden contraction inlet critical Reynolds numbers were found to be between 1 800 and 2 000 for adiabatic cases, while for diabatic cases the transition regime commenced at a Reynolds number of about 2 000. The bellmouth and swirl inlet types were investigated for diabatic cases only with swirl inlet tests limited to the 1.05 mm channel. Laminar 2 friction factors were approximately similar to those of the sudden contraction inlet type, however, after the commencement of transition both inlet types exhibited higher friction factors than the sudden contraction inlet. Minor transition occurred as early as at Reynolds numbers of 1 200 and 800 for the bellmouth and swirl inlet types respectively while major transition occurred at Reynolds numbers of approximately 1 800 and 1 500 respectively. Critical Reynolds numbers were found not be significantly influenced by the channel to diameter to length ratio considered in this study. Laminar Nusselt numbers were predicted well by conventional macro-channel thermal entry correlations. The swirl inlet type exhibited the highest friction factors and Nusselt numbers in the transitional regime followed by the bellmouth inlet type. During transition while compared with the sudden contraction inlet, both the bellmouth and swirl inlet types exhibited larger enhancement in heat transfer than increases in the friction factor penalty. Based on the experimental data obtained in this study, a set of correlations were developed which describes the relation between the friction factor and Colburn jfactor. Depending on the inlet type, the correlations predicted between 94% and 100% of the results to within 10% of the experimental measurements.

Section: Bellmouth Inlet Resultscontrasting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Inlet flow effects in micro-channels in the laminar and transitional regimes on single-phase heat transfer coefficients and friction factors

Dirker

International Journal of Heat and Mass Transfer

Garach

2014

“…The smoother the inlet, the more transition is delayed. Results published in later work by Tam and Ghajar (1997) showed transition to occur at different Reynolds numbers (much higher) than previous results. The transition for the re-entrant inlet started and ended at 2900 to 3500, at 3100 to 3700 for the square-edged, and at 5100 to 6100 for the bellmouth.…”

Section: Introductionmentioning

confidence: 48%

Single-Phase Heat Transfer and Pressure Drop of the Cooling of Water inside Smooth Tubes for Transitional Flow with Different Inlet Geometries (RP-1280)

Olivier¹,

2010

HVAC&R Research

“…Here the experimental data is under predicted by an average of 8.5% (however, it is within the uncertainties of 18% at low Reynolds numbers) with a maximum deviation of 2.2%. According to Shome and Jensen [34] and Tam and Ghajar [35], secondary flow effects increase the friction factor, especially in tubes with uniform heat flux boundary conditions.…”

Section: The Delayed Transition and The Sharp Transition Is The Resulmentioning

confidence: 99%

“…Tam and Ghajar [35] discovered that by increasing the overall heat flux, the laminar friction factors increased. The reason for this is that the wall-to-bulk temperature difference exists throughout the length of the tube [34].…”

Section: Diabatic Friction Factorsmentioning

confidence: 99%

The influence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional flow regime of smooth tubes

International Journal of Heat and Mass Transfer

McKrell

Grote

2013

111

In this paper, the convective heat transfer enhancement of aqueous suspensions of multiwalled carbon nanotubes flowing through a straight horizontal tube was investigated experimentally for a Reynolds number range of 1 000 -8 000, which included the transitional flow regime. The tube was made out of copper with an internal diameter of 5.16 mm.Experiments were conducted at a constant heat flux of 13 kW/m 2 with 0.33%, 0.75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10 -20 nm, an inside diameter of 3 -5 nm and a length of 10 -30 µm.Temperature and pressure drop measurements were taken, from which the heat transfer coefficients and friction factors were determined as a function of Reynolds number. It was found that heat transfer was enhanced when comparing the data on a Reynolds-Nusselt graph but when comparing the data at the same velocity, it was shown that heat transfer was not enhanced. Performance evaluation of the nanofluids showed that the increase in viscosity was four times the increase in the thermal conductivity, which resulted in an inefficient nanofluid.