An investigation was conducted to evaluate the impact of TiO2/water nanofluids on the cooling system of a central processing unit (CPU). Tests were carried out using nanofluids with TiO2 concentrations of 0.02%, 0.05%, 0.1%, and 0.15% by volume at a Reynolds number of 20,000. Pure water (as the base fluid) was also tested for comparative analysis. The experimental findings emphasized the significant influence of nanofluid application on CPU cooling. Under identical operating conditions, the heat transfer rate of TiO2/water nanofluids proved more effective compared to pure water. Moreover, the results indicated that the nanofluid with a 0.05% volume concentration reduced CPU temperature by 1%, 0.7%, and 1.5% compared to TiO2 concentrations of 0.02%, 0.1%, and 0.15% by volume, respectively. At the optimal condition, the TiO2/water nanofluid with a volume concentration of 0.05% exhibited the highest Nusselt number, leading to a 2.4% decrease in CPU temperature compared to the base fluid.

Convective heat transfer by impinging air jets was investigated experimentally using a thermochromic liquid crystal (TLC) sheet. The effects of jet-to-plate spacings (1.0 <= L/D <= 8.0) and nozzle shapes (circular and rectangular nozzles), were examined at constant Reynolds number Re = 20,000 (based on the jet nozzle diameter). The experimental results showed that average Nusselt number (Nu) increased with decreasing jet-to-plate spacing (L/D), due to a higher impingement velocity. At same jet-to-plate spacing, a square air jet gave higher Nusselt number than a circular one. The superior heat transfer of the square air jet can be attributed to a turbulence/recirculation effect at four corners of the nozzle.

This paper presents the numerical study of the turbulent flow and heat transfer in the round tubes equipped with multi-channel twisted tapes (MC-TT). A finite volume method with the RNG k- ε turbulence model was applied for the simulation. The effects of the twist ratio ( defined as a ratio of twist length to tape width: y/w = 1.5, 2.0, 3.0, and 4.0) and number of channels (N = 2 and 4) on the fluid flow and heat transfer characteristics were investigated in a turbulent flow regime (5000 ≤ Re ≤ 15000). The computations show that the circular tubes with multi-channel twisted tapes give higher heat transfer rate than the plain tube by around 22-30%. Heat transfer enhancement by the multi-channel twisted tapes is strongly dependent on twist ratio (y/w) and number of channel (N). For the present study, the tape with N = 2 with twist ratio, y/w = 3.0 offers the best heat transfer enhancement with the maximum thermal performance factor of 1.02.

This paper presents a study on heat transfer enhancement by swirling impinging jets. The swirling jets were induced by inserting dual twisted tapes into jet nozzles. The effects of jet Reynolds number (4000<Re<16,000), jet-to-plate spacing (2<L/D<8), tape twist ratio (y/W = 3, 4, 5, 6) and swirling direction were experimentally investigated. Numerical visualization was also carried out to study a behavior of the flow phenomena of swirling jets.

Heat transfer augmentation by dual-helical twisted tapes (D-HTTs) and triple-helical twisted tapes (T-HTTs) is presented in comparison with that by single-helical twisted tape (S-HTT). The effects of tape width ratio (w/D = 0.1, 0.15, and 0.2) on heat transfer, friction factor, and thermal performance are also reported. The experiments were performed using air as a working fluid for Reynolds number between 6000 and 20,000. At similar conditions, the use of D-HTTs and T-HTTs leads to the increase of Nusselt number by respectively 15.6% to 17.6% and 19.5% to 23.4%, the increase of friction factor by respectively 83% to 206% and 143% to 335%, and the decrease of thermal performance factor by 3.9% to 20.3% and 8.3% to 26.2% compared to those obtained from the use of S-HTT/promoter. As the width ratio increases, both Nusselt number and friction factor are considerably increased. However, due to the poorer trade-off between the increases of Nusselt number and friction factor, lower thermal performance factor is consistently obtained at the conditions possessing higher Nusselt number. At similar conditions, S-HTTs consistently give a higher thermal performance factor than D-HTTs and T-HTTs; the maximum thermal performance factors given by S-HTTs with w/D of 0.1, 0.15, and 0.2 are 1.34, 1.31, and 1.29, respectively (at Reynolds number of 6000).

Turbulent flow characteristics and heat transfer performance of heat exchanger tube fitted with annulus circular-rings (A-CRs) are experimentally studied. The d/D_o = 0.1, 0.15 and 0.2 were inserted into a round tube at constant space length ratio (s/D_o) of 1.0. For optimization, the A-CRs at different annulus diameter ratios of d/D_o = 0.1, 0.15 and 0.2 were comparatively tested. The experimental data of the tube with A-CRs are compared with those of the tube with a typical circular-rings (CRs) and also the plain tube. The results show that using the A-CRs with d/D_o = 0.1, 0.15 and 0.2 improves heat transfer rates by around 210%, 244% and 303%, respectively over that of the plain tube. The enhanced heat transfer rates are respectively accompanied with the increased friction factors of around 28, 47 and 90 times of the plain tube. Both friction factor and heat transfer caused by the A-CRs are considerably higher than those caused by the CRs. However, the tubes with A-CRs yield lower thermal performance factors than the one with CRs. In order to understand the heat transfer enhancement mechanism of the heat exchanger tube fitted with A-CR, the numerical results of streamlines and fluid temperature distributions are also reported.

The influence of helical screw tape coupled with rib turbulators (HST-R) on thermohydraulic characteristics in a circular tube has been experimentally investigated. The experiments were performed under constant heat flux condition for Reynolds number between 6,000 and 20,000. The rib-pitch ratio (p/D) was varied from 1.0 to 3.0 while rib-height ratio (e/W) was varied from 0.5 to 1.5. The experimental results revealed that heat transfer and friction factor increase with increasing rib-height ratio (e/W) and decreasing rib-pitch ratio (p/D). For the range examined, the HST-Rs with moderate rib-pitch ratio (p/D = 2) and the largest rib-height ratio (e/W = 1.5) gave the maximum thermal performance factor while HST possessed thermal performance factor around average value of those of the tubes with HSTRs. The developed empirical correlations for Nusselt number, friction factor and thermal performance factor gave the predictions within ±4.4%, ±13% and ±4.2%, respectively as compared to the experimental data.

The influences of circular-ring turbulators (CRT) and twisted tape (TT) swirl generators on the heat transfer enhancement, pressure drop and thermal performance factor characteristics in a round tube are reported. The circular-ring turbulators were individually employed and together with the twisted tape swirl generators in the heated section of the tube. Three different pitch ratios (l/D = 1.0, 1.5, and 2.0) of the CRT and three different twist ratios (y/W = 3, 4, and 5) of the TT were introduced. The experiments were conducted using air as the working fluid under a uniform wall heat flux condition, for the Reynolds number between 6000 and 20000. The experimental results reveal that the heat transfer rate, friction factor and thermal performance factor of the combined CRT and TT are considerably higher than those of CRT alone. For the range examined, the increases of mean Nusselt number, friction factor and thermal performance, in the tube equipped with combined devices, respectively, are 25.8%, 82.8% and 6.3% over those in the tube with the CRT alone. The highest thermal performance factor of 1.42 is found for the combined device consisting of the CRT with l/D = 1.0 and TT with y/W = 3. The correlations of the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices are also developed in terms of Reynolds number, Prandtl number, twist ratio and pitch ratio.

This paper presents the heat transfer augmentation and friction factor characteristics by means of dimpled tubes. The experiments were conducted using the dimpled tubes with two different dimpled-surface patterns including aligned arrangement (A-A) and staggered arrangement (S-A), each with two pitch ratios (PR = p/Di = 0.6 and 1.0), for Reynolds number ranging from 9800 to 67,000. The experimental results achieved from the dimpled tubes are compared with those obtained from the plain tube. Evidently, the dimpled tubes with both arrangements offer higher heat transfer rates compared to the plain tube and the dimpled tube with staggered arrangement shows an advantage on the basis of heat transfer enhancement over the dimpled tube with aligned arrangement. The increase in heat transfer rate with reducing pitch ratio is due to the higher turbulent intensity imparted to the flow between the dimple surfaces. The mean heat transfer rate offered by the dimpled tube with staggered arrangement (S-A) at the lowest pitch ratio (PR = 0.6), is higher than those provided by the plain tube and the dimpled tube with aligned arrangement (A-A) at the same PR by around 127% and 8%, respectively. The empirical correlations developed in terms of pitch ratio (PR), Prandtl number (Pr) and Reynolds number, are fitted the experimental data within ±8% and ±2% for Nusselt number (Nu) and friction factor (f), respectively. In addition, the thermal performance factors under an equal pumping power constraint of the dimple tubes for both dimpled-surface arrangements are also determined.

Experimental investigation of heat transfer and friction factor behaviors in a rectangular channel with multiple twisted tapes (MTTs) is carried out. In each run, five MTTs are simultaneously used as the swirl flow generators in a test section. The lower wall of the channel is subjected to uniform heat flux conditions while the upper wall is insulated. Parametric runs are made for Reynolds numbers ranging from 2700 to 9000, the tapes with twist-length to twist-width ratios (y/w) of 3, 4 and 5 while the aspect ratio of AR = W/H = 10 is fixed throughout. Experiments using smooth channel are also performed for comparison. The experimental results reveal that heat transfer in the channels with MTTs at twist ratios (y/w) of 3, 4 and 5 are respectively increased around 55%, 37% and 22% over those in the smooth channel. In addition, the empirical correlations of the Nusselt number and friction factor in the developed flow region for the channels fitted with MTTs are also derived.