Passive Heat Transfer Enhancement Techniques Research Articles

THERMO-HYDRAULIC BEHAVIOR MODELING OF PASSIVE HEAT TRANSFER ENHANCEMENT TECHNIQUES USING A SOFT COMPUTING APPROACH

The main aim of this research is to demonstrate effectiveness of soft computing techniques in thermo-hydraulic behavior modeling of passive heat transfer enhancement (HTE) techniques. An artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS), two effective modeling methods, have been used to model Nusselt numbers and friction factors of wire coil and twisted tape inserts in various flow regimes. The experimental data sets were utilized for training and validation of these models, and their results were compared with the corresponding correlations. The mean relative error (MRE) between the predicted results and experimental data of ANN and ANFIS models were found to be less than 3% and 1.5% for thermo-hydraulic behavior modeling of wire coil and twisted tape inserts, respectively. Depending on model complexity, performance of both ANN and ANFIS models was found to be superior to that of the corresponding power-law regressions. Hence, application of the soft computing approach to predict the performance of thermal systems in engineering applications is recommended.

Numerical Study of Fluid Flow and Heat Transfer in the Enhanced Microchannel With Oblique Fins

Sectional oblique fins are employed in contrast to continuous fins in order to modulate flow in microchannel heat sink. The breakage of continuous fin into oblique sections leads to the reinitialization of both hydrodynamic and thermal boundary layers at the leading edge of each oblique fin, effectively reducing the thickness of boundary layer. This regeneration of entrance effect causes the flow to be always in a developing state thus resulting in better heat transfer. In addition, the presence of smaller oblique channels diverts a small fraction of flow into the adjacent main channels. The secondary flows thus created improve fluid mixing which serves to further enhance the heat transfer. Detailed numerical study on the fluid flow and heat transfer of this passive heat transfer enhancement technique provides insight to the local hydrodynamics and thermal development along the oblique fin. The uniquely skewed hydrodynamic and thermal profiles are identified as the key to the highly augmented and uniform heat transfer performance across the heat sink. The associated pressure drop penalty is much smaller than the achieved heat transfer enhancement, rendering it as an effective heat transfer enhancement scheme for single phase microchannel heat sink.

Effect of Geometric and Operating Parameters on Performance of Helical Coiled Tube Heat Exchanger

Helical coil heat exchangers are popular because of compact structure with enhanced heat transfer coefficient by passive heat transfer enhancement technique. Due to these advantages helical coil tube heat exchangers are widely used in various applications like power plants, nuclear reactors, refrigeration and air-conditioning systems, heat recovery systems, chemical processing and food industries. The analysis of the helical coil heat exchanger with different operating and geometric parameters is presented in this paper. The analysis shows that both operating and geometric parameters are having influence on heat transfer characteristic. The analysis of heat transfer coefficient and Nusselt Number with respect to Reynolds number with different correlations and its signifence is discussed in this paper. The effect of geometric parameter on performance of helical coiled tube heat exchanger is also presented in this paper.

Compound convective heat transfer enhancement in helically coiled wall corrugated tubes

The objective of the research was to investigate the passive compound heat transfer enhancement technique achievable by combining the effect of wall curvature and of wall corrugation. To this aim, the forced convective heat transfer in helically coiled smooth and corrugated wall tubes was studied in the Reynolds and Dean number ranges 70÷1200 and 12÷290 respectively, by adopting Ethylene Glycol as working fluid. Three different curvature ratio values were considered i.e. 0.031, 0.040 and 0.056, while the wall corrugation profile was held constant. For low De values the wall curvature effect prevailed, and the heat transfer enhancement was nearly the same for the corrugated and the smooth helically coiled tubes. For higher De values instead, the wall corrugation brought a further heat transfer enhancement with respect to the wall curvature. This behavior could be explained as a consequence of an early departure from the steady laminar flow regime, as it happens in straight wall corrugated tubes. With this regard a critical Dean number value equal to 120 was identified. The results showed that the effect of the wall curvature alone brings heat transfer enhancement in the range 2÷10 over the straight smooth wall tube against pressure drop penalties augmentation in the range 1÷1.5, while for the helical coiled wall corrugated tubes an heat transfer augmentation over the straight smooth wall behavior of up to 25 was registered against a maximum friction factor increase of 2.5. The present research highlights that the simultaneous use of the two passive heat transfer enhancement techniques (wall curvature compounded with wall corrugation) is particularly effective if compared to the performance achievable through their separate use.

Experimental investigation on the convective heat transfer in straight and coiled corrugated tubes for highly viscous fluids: Preliminary results

The forced convective heat transfer in straight and coiled tubes, having smooth and corrugated wall, was experimentally investigated in two ranges of the Reynolds number: a lower one (5 < Re < 13) obtained with Glycerol and a higher one (150 < Re < 1500) obtained with Ethylene Glycol. The aim of the research was to verify the effectiveness of these passive heat transfer enhancement techniques when highly viscous fluids are treated. This issue is particularly crucial in applications in which the thermal processing of high Prandtl number fluids is required, such as in the food, chemical, pharmaceutical and cosmetics industries. In the present note, preliminary results, obtained by considering a given geometrical configuration characterized by a tube diameter of 14 mm, a curvature ratio of 0.06, a corrugation depth of 1 mm and a corrugation pitch of 16 mm, are presented. The main conclusion is that the wall curvature enhances heat transfer at all Re, whereas the wall corrugation enhances heat transfer only in the higher Re range; moreover the wall corrugation is totally ineffective in the low Re range and, if helical coils are present, it also destroys the benefit induced by the wall curvature. The largest increment in heat transfer rates is thus obtained by using smooth helical coils at low Re, and corrugated helical coils at larger Re. The results, although of preliminary nature, suggest interesting applications of the passive heat transfer enhancement technique based on smooth wall coiled tubes in the very low Reynolds number values range, while the combined passive technique based on wall corrugation and curvature represents an interesting solution for Reynolds number values in the range 150–1500.

Numerical investigation of heat transfer and pressure drop in enhanced tubes

This study investigates passive heat transfer enhancement techniques to determine the distribution of temperature and static pressure in test tubes, the friction factor, the heat flux, the temperature difference between the inlet and outlet fluid temperatures, the pressure drop penalty and the numerical convective heat transfer coefficient, and then compares the results to the experimental data of Zdaniuk et al. It predicts the single-phase friction factors for the smooth and enhanced tubes by means of the empirical correlations of Blasius and Zdaniuk et al. This study performed calculations on a smooth tube and two helically finned tubes with different geometric parameters also used in the analyses of Zdaniuk et al. It also performed calculations on two corrugated tubes in the simulation study. In Zdaniuk et al.'s experimental setup, the horizontal test section was a 2.74 m long countercurrent flow double tube heat exchanger with the fluid of water flowing in the inner copper tube (15.57–15.64 mm i.d.) and cooling water flowing in the annulus (31.75 mm i.d.). Their test runs were performed at a temperature around 20 °C for cold water flowing in the annulus while Reynolds numbers ranged from 12,000 to 57,000 for the water flowing in the inner tube. A single-phase numerical model having three-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the flow. The temperature contours are presented for inlet, outlet and fully developed regions of the tube. The variations of the fluid temperature and static pressure along tube length are shown in the paper. The results obtained from a numerical analysis for the helically tubes were validated by various friction factor correlations, such as those found by Blasius and Zdaniuk et al. Then, numerical results were obtained for the two corrugated tubes as a simulation study. The present study found that the average deviation is less than 5% for the friction factors obtained by the Fluent CFD program while Blasius's correlation has the average deviation of less than 10%. The corrugated tubes have a higher heat transfer coefficient than smooth tubes but a lower coefficient than helically finned tubes. The paper also investigates the pressure drop penalty for the heat transfer enhancement.

Investigation on the jet liquid impingement heat transfer for the central processing unit of personal computers

In this paper, the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink for the central processing unit of a personal computer are experimentally investigated. The experiments are tested with three different channel width heat sinks under real operating conditions: no load and full load conditions. The jet liquid impingement cooling with mini-rectangular fin heat sink system is introduced as the active and passive heat transfer enhancement techniques. Effects of relevant parameters on the central processing unit temperature are considered. It is found that the central processing unit temperatures obtained from the jet liquid impingement cooling system are lower than those from the conventional liquid cooling system; however, the energy consumption also increases. The results of this study are of technological importance for the efficient design of cooling systems of personal computers or electronic devices to enhance cooling performance.

95/06455 Passive heat transfer enhancement techniques applied to compact bubble absorber design

The thermodynamic behavior and lack of ozone-depleting potential of 1,1,1,2-tetrafluoroethane (R-134a) suggest it as a likely replacement for dichlorodifluoromethane (R-12), now used as the refrigerant in many air-conditioning systems. To further the presently incomplete toxicological analysis of R-134a, the effects of R-134a on cell viability and functional competence of glucose metabolism were evaluated in suspension cultures of hepatocytes derived from fed or fasted rats. R-134a concentrations up to and including 75% (750,000 ppm) in the gas phase of sealed culture flasks did not produce evidence of cytolethality (LDH leakage) following 2 hr of exposure; in contrast, halothane (1,1,1-trifluoro-2-bromo-2-chloroethane) caused cell death at a gas phase concentration of only 1250 ppm. In hepatocytes isolated from fed rats, R-134a at concentrations of 12.5 to 75% increased glycolysis (production of lactate + pyruvate) in a concentration-dependent manner, no effect was observed at 5%. At 25%, R-12 and 1,1,2,2-tetrafluoro-1,2-dichloroethane (R-114) were of equal potency to R-134a in stimulating glycolysis; 1,1,1,2,2-pentafluoro-2-chloroethane (R-115) depressed glycolysis slightly. Halothane, at concentrations as low as 300 ppm, markedly increased rates of glycolysis. Glucose production by hepatocytes of fed rats was decreased by R-134a, R-12, and R-114 only at concentrations of 25% or more. On the other hand, halothane (≥300 ppm) potently decreased glucose production by hepatocytes. In cells isolated from livers of fasted rats, R-134a exposure inhibited gluconeogenesis in a concentration-dependent manner although this effect was not significant until R-134a concentrations reached 12.5%. Comparative potency studies showed that R-134a, R-12, or R-114 (25% gas phase) inhibited gluconeogenesis about equally while as little as 300 ppm halothane was effective and R-115 (25%) was without effect. Considering that the threshold for alteration of the rate of glucose metabolism in this in vitro paradigm is about 12.5% R-134a, we conclude that toxicologically significant alteration of glucose-linked bioenergetics is unlikely at the levels of R-134a exposure anticipated in workplace or environment.