Air-Cooling Enhancement with Delta Winglet Vortex Generators in Entrance Region of In-Line Array Electronic Modules

The objective of this study is to experimentally investigate the heat transfer enhancement by delta winglet vortex generators for air cooling in the entrance region of an in-line array of electronic modules. The study has been carried out when the winglet pairs are placed in front of all modules with attack angles of 10, 15 and 20 degrees. Each module has dimensions of 1.8 cm × 5.8 cm × 0.6 cm and generates heat at 5 W. The adiabatic heat transfer coefficients and the thermal wake functions for the modules with and without the generators are considered at different values of Reynolds number. It could be seen that the vortex generators could enhance the adiabatic heat transfer coefficients and reduce the thermal wake function and the module temperature significantly. Moreover, the correlations to predict the heat transfer data when the vortex generators are integrated have been developed. The temperatures of the modules in each row could be predicted from these correlations, and the results agree very well with the experimental data.     

Experimental study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.     

Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators

The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y = 4 and 5), three winglet- to duct-height ratios, (R_B = 0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R_P = 2, 2.5, 4 and 5) and a single attack angle of winglet, α = 30°. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing R_B but decreasing R_P. The inserted duct at R_B = 0.2, R_P = 2 and Y = 4 provides the highest heat transfer rate and friction factor but the one at R_B = 0.1, R_P = 2 and Y = 4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone.     

Numerical study of heat-transfer enhancement by punched winglet-type vortex generator arrays in fin-and-tube heat exchangers

The potential of punched winglet type vortex generator (VG) arrays used to enhance air-side heat-transfer performance of finned tube heat exchanger is numerically investigated. The arrays are composed of two delta-winglet pairs with two layout modes of continuous and discontinuous winglets. The heat transfer performance of two array arrangements are compared to a conventional large winglet configuration for the Reynolds number ranging from 600 to 2600 based on the tube collar diameter, with the corresponding frontal air velocity ranging from 0.54 to 2.3 m/s. The effects of different geometry parameters that include attack angle of delta winglets (β = 10 deg, β = 20 deg, β = 30 deg) and the layout locations are examined. The numerical results show that for the punched VG cases, the effectiveness of the main vortex to the heat transfer enhancement is not fully dominant while the “corner vortex” also shows significant effect on the heat transfer performance. Both heat transfer coefficient and pressure drop increase with the increase of attack angle β for the side arrangements; the arrays with discontinuous winglets show the best heat transfer enhancement, and a significant augmentation of up to 33.8–70.6% in heat transfer coefficient is achieved accompanied by a pressure drop penalty of 43.4–97.2% for the 30 deg case compared to the plain fin. For the front arrangements of VGs higher heat transfer enhancement and pressure drop penalty can be obtained compared to that of the side arrangement cases; the case with front continuous winglet arrays has the maximum value of j/f, a corresponding heat transfer improvement of 36.7–81.2% and a pressure drop penalty of 60.7–135.6%.     

The development of active vortex generators from shape memory alloys for the convective cooling of heated surfaces

A study of the convective heat transfer enhancement of heated surfaces through the use of active delta wing vortex generators is reported in this paper. The surface-mounted vortex generators (VGs) change their shape to intrude further into the flow at high temperatures to enhance heat transfer, while maintaining a low profile at low temperatures to minimise flow pressure losses. The VGs are made from shape memory alloys and manufactured in a selective laser melting process. Experiments have been carried out in a rectangular duct supplied with laminar-transition air flow. In the test section, a single, and a pair of active delta wing VGs were placed near the leading edge of a heated plate and tested separately for their heat transfer enhancement effects using infrared thermography. The pressure difference across the test section was also measured to determine the pressure drop penalty associated with the obstruction caused by the vortex generators in their active positions. Promising shape memory response was obtained from the active VG samples when their surface temperatures were varied from 20 °C to 65 °C. The vortex generators responded by increasing their angles of attack from 10° to 38° and as the designs were two-way trained, they regained their initial position and shape at a lower temperature. At their activated positions, maximum heat transfer improvements of up to 90% and 80% were achieved by the single and double wings respectively along the downstream direction. The flow pressure losses across the test section, when the wings were activated, increased between 7% and 63% of the losses at their de-activated positions, for the single and double VG respectively.     

Heat transfer study in a discoidal system: The influence of rotation and space between disks

This article presents an experimental study of the local heat transfer on the rotor surface in a discoidal rotor–stator system air-gap in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance was used to identify the local convective heat transfer coefficient. The influence of the dimensionless spacing interval G between the disks and of the rotational Reynolds number Re was measured and compared with the data available in bibliography. Local convective heat transfer coefficients were obtained for an axial Reynolds number Re_j = 41.6 × 10³, a rotational Reynolds number Re between 0.2 × 10⁵ and 5.16 × 10⁵, and a dimensionless spacing interval G ranging from 0.01 to 0.16.     

Effects of duct inclination angle on thermal entrance region of laminar and transition mixed convection

Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from ?90° to 90°. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (θ = ? 90°, ?60°, ?30°, 0°, 30°, 60° and 90°), six Reynolds numbers (Re ≈ 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 × 10³ to 4.1 × 10⁴. The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30° to ?30° with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from ?90° to 90° when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers.     

Energy separation in the wake of a cylinder: Effect of Reynolds number and acoustic resonance

Energy separation is a spontaneous redistribution of total energy in a flowing fluid without external work or heat transfer. The energy separation mechanism in the vortex field behind an adiabatic circular cylinder in a cross flow of air is investigated. Time-averaged velocity and temperature measurements taken one diameter downstream of the cylinder (Re ～ 10⁵, M_∞ ～ 0.25) indicate flow reversal. The measured recovery temperature, expressed as distribution of energy separation factor indicates that energy separation is caused by the vortex flow in the wake, enhanced by acoustic excitation, and is insensitive to Reynolds number in the sub critical range studied.     

Numerical modeling of heat and mass transfer during forced convection drying of rectangular moist objects

A two-dimensional analysis of heat and mass transfer during drying of a rectangular moist object is performed using an implicit finite difference method, with the convective boundary conditions at all surfaces of the moist object. The variable convective heat and mass transfer coefficients are considered during the drying process. The external flow and temperature fields are first numerically predicted through the Fluent CFD package. From these distributions, the local distributions of the convective heat transfer coefficients are determined, which are then used to predict local distributions of the convective mass transfer coefficients through the analogy between the thermal and concentration boundary layers. Also, the temperature and moisture distributions for different periods of time are obtained using the code developed to determine heat and mass transfer inside the moist material. Furthermore, the influence of the aspect ratio on the heat and mass transfer is studied. It is found that the convective heat transfer coefficient varies from 4.33 to 96.16 W/m² K, while the convective mass transfer coefficient ranges between 9.28 × 10⁻⁷ and 1.94 × 10⁻⁵ m/s at various aspect ratios. The results obtained from the present analysis are compared with the experimental data taken from the literature, and a good agreement is observed.     

Experimental investigation of mixed convection heat transfer in a rectangular channel with discrete heat sources at the top and at the bottom

Mixed convection heat transfer in a top and bottom heated rectangular channel with discrete heat sources has been investigated experimentally for air. The lower and upper surfaces of the channel were equipped with 8 × 4 flush-mounted heat sources subjected to uniform heat flux. Sidewalls, the lower and upper walls were insulated and adiabatic. The experimental study was made for an aspect ratio of AR = 6, Reynolds numbers 955 ≤ Re_Dh ≤ 2220 and modified Grashof numbers Gr* = 1.7 × 10⁷ to 6.7 × 10⁷. From experimental measurements, surface temperature and Nusselt number distributions of the discrete heat sources were obtained for different Grashof numbers. Furthermore, Nusselt number distributions were calculated for different Reynolds numbers. Results show that surface temperatures increase with increasing Grashof number. The row-averaged Nusselt numbers first decrease with the row number and then, due to the increase in the buoyancy affected secondary flow and the onset of instability, they show an increase towards the exit as a result of heat transfer enhancement.     

Three-dimensional numerical study on fin-and-oval-tube heat exchanger with longitudinal vortex generators

Three-dimensional numerical study was performed for heat transfer characteristics and fluid flow structure of fin-and-oval-tube heat exchangers with longitudinal vortex generators (LVGs). For Re (based on the hydraulic diameter) ranges from 500 to 2500, it was found that the average Nu for the three-row fin-and-oval-tube heat exchanger with longitudinal vortex generators increased by 13.6–32.9% over the baseline case and the corresponding pressure loss increased by 29.2–40.6%. The results were analyzed on the basis of the field synergy principle to provide fundamental understanding of the relation between local flow structure and heat transfer augmentation. It was confirmed that the reduction of the intersection angle θ between the velocity field and the temperature field was one of the essential factors influencing heat transfer enhancement. Three geometrical parameters – placement of LVGs (upstream and downstream), angles of attack (α = 15°, 30°, 45° and 60°) and tube-row number (n = 2, 3, 4 and 5) – were also investigated for parameter optimization. The LVGs with placement of downstream, angles of attack α = 30° and minimum tube-row number provide the best heat transfer performance. The effects of the three geometrical parameters on heat transfer enhancement were also analyzed from the view point of the field synergy principle and it was found that the results can be well explained by the field synergy principle.     

Numerical study on laminar convection heat transfer in a channel with longitudinal vortex generator. Part B: Parametric study of major influence factors

This paper presents the influences of main parameters of longitudinal vortex generator (LVG) on the heat transfer enhancement and flow resistance in a rectangular channel. The parameters include the location of LVG in the channel, geometric sizes and shape of LVG. Numerical results show that the overall Nusselt number of channel will decrease with the LVGs’ location away from the inlet of the channel, and decrease too with the space between the LVG pair decreased. The location of LVG has no significant influence on the total pressure drop of channel. With the area of LVG increased, the average Nusselt number and the flow loss penalty of channel, especially when β = 45° will increase. With the area of LVG fixed, increasing the length of rectangular winglet pair vortex generator will bring about more heat transfer enhancement and less flow loss increase than that increasing the height of rectangular winglet pair vortex generator. With the same area of LVG, delta winglet pair is more effective than rectangular winglet pair on heat transfer enhancement of channel, and delta winglet pair-b is more effective than delta winglet pair-a. Delta winglet pair-a results in a higher pressure drop, the next is rectangular winglet pair and the last is delta winglet-b. The increase of heat transfer enhancement is always accompanied with the decrease of field synergy angle between the velocity and temperature gradient when the parameters of LVG are changed. This confirms again that the field synergy is the fundamental mechanism of heat transfer by longitudinal vortex. The laminar heat transfer of the channel with punched delta winglet pair is experimentally and numerically studied in the present paper. The numerical result for the average heat transfer coefficient of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.     

Experimental study on thermal characteristics of suspended platinum nanofilm sensors

In this paper, the thermal characteristics of suspended platinum (Pt) nanofilm sensors have been investigated experimentally. The Pt nanofilm sensors with the thickness of 28–40 nm, the width of 260–601 nm, and the length of 5.3–5.7 μm were fabricated by electron beam lithography, electron beam physical vapor deposition and isotropic/anisotropic etching processes. Based on the one-dimensional heat conduction model, the in-plane thermal conductivity of the nanofilm sensors was obtained from the linear relation of the volume-averaged temperature increase and the heating rate measured in vacuum. Furthermore, natural convection heat transfer coefficients of air around the suspended nanofilm sensors at the pressures ranging from 1 × 10⁻² Pa to 1 atm were also investigated. The experimental results show that the in-plane thermal conductivities of the nanofilm sensors are much lower than those of the bulk values, the natural convection heat transfer coefficients are, however, very high at the atmospheric pressure.     

Regularities of flow and heat transfer at the surface of transversely finned tubes

A great number of experimental investigations allowing one to reveal the physical mechanism of processes responsible for their thermal and hydraulic performance are carried out in attempt to solve problems of updating constructions and methods of thermal design of heating surfaces of transversely finned tubes widespread in power engineering. Results of flow visualization and investigation of pressure fields and local heat transfer at the fin surface over the Reynolds number range Re = (1.0 ? 6.6) · 10⁴ are presented for the case of a wide variation of geometric characteristics of finned tubes and parameters of their arrangement in a bundle. Regularities substantially changing the existing concept of transfer processes in the interfin space and in the wake behind a finned tube are revealed. It is found that the flow behavior and the distribution of local heat transfer coefficients over the fin surface change significantly at the fin height-to-finned tube diameter h/d approximately equal to 0.4. The results obtained are generalized in the form of the patterns of flow and heat transfer at the finned tube surface, including seven characteristic regions and four types of flow separation.     

New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room

In this study, radiative and convective heat transfer coefficients at the ceiling are determined for a cooled ceiling room. Firstly, convective heat transfer is simulated numerically neglecting the radiative heat transfer at the surfaces (ε_f = ε_w = ε_c = 0), then, radiative heat transfer is calculated theoretically for different surface emissivities (ε_f = ε_w = ε_c = 0.5, 0.6, 0.7, 0.8 and 0.9) for different room dimensions (3 × 3 × 3, 4 × 3 × 4 and 6 × 3 × 4 m) and thermal conditions (T_f = 25 °C, T_w = 28–36 °C and T_c = 0–25 °C). Numerical data is compared with the results of correlations based on experimental data given in literature. New equations related to convective and total (including the effect of convection and radiation) heat transfer coefficients for ceiling are found in the current study.     

Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array

In this paper, overall thermal energy and exergy analysis has been carried out for different configurations of hybrid photovoltaic thermal (PVT) array. The hybrid PVT array (10.08 m × 2.16 m) is a series and parallel combinations of 36 numbers of PV modules. A one-dimensional transient model for hybrid PVT array has been developed using basic heat transfer equations. On the basis of this transient model, an attempt has been made to select an appropriate hybrid PVT array for different climatic conditions (Bangalore, Jodhpur, New Delhi, and Srinagar) of India. On the basis of high grade energy (i.e. overall exergy gain), case-III has been selected as the most appropriate configuration because overall exergy for case-III is 12.9% higher than case-II. The overall thermal energy and exergy gain for Bangalore is 4.54 × 10⁴ kW h and 2.07 × 10⁴ kW h respectively which is highest in comparison to the other cities.     

Orientation effect on heat transfer of a shrouded LED backlight panel with a plate-fin array

This study reports thermal performance of a shrouded 348 mm × 558 mm aluminum plate-fin heat sink subject to various input powers and orientations. Effects of clearance (C) and the orientation on the heat transfer of the heat sink were investigated. Results show that the clearance effect is detectable only in a “window region” between 5 mm and 10 mm where an appreciable rise of heat transfer coefficient is encountered. As the tilted angle (θ) of the LED panel is increased, the heat transfer coefficient is reduced and the clearance effect on heat transfer becomes more pronounced. The heat transfer coefficients are similar between two cases in which the tilted angles of the LED panel are supplementary irrespective of clearance and input power. Except the cases of a horizontal heat sink, heat transfer coefficient of the shrouded heat sink having a fin array facing downward is usually slightly higher than that having supplementary tilted angle.     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer

In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 °C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 × 12.5 × 25 mm) and cylindrical shaped pumpkin slices (35 × 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m² C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.     

Investigation of mixed convection heat transfer in a horizontal channel with discrete heat sources at the top and at the bottom

Mixed convection heat transfer from arrays of discrete heat sources inside a horizontal channel has been investigated experimentally. Each of the lower and upper surfaces of the channel was equipped with 8 × 4 flush mounted heat sources subjected to uniform heat flux. Sidewalls, lower and upper walls are insulated and adiabatic. The experimental parametric study was made for aspect ratios of AR = 2, 4 and 10, at various Reynolds and Grashof numbers. From the experimental measurements, row-average surface temperature and Nusselt number distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these numbers were investigated. From these results, the buoyancy affected secondary flow and the onset of instability have been discussed. Results show that top and bottom heater surface temperatures increase with increasing Grashof number. The top heater average-surface temperatures for AR = 2 are greater than those of bottom ones. For high values of Grashof numbers where natural convection is the dominant heat transfer regime (Gr¹/Re² ≫ 1), temperatures of top heaters can have much greater values. The variation of the row-average Nusselt numbers for the aspect ratio of AR = 4, show that with the increase in the buoyancy affected secondary flow and the onset of instability, values of Nusselt number level off and even rise as a result of heat transfer enhancement especially for low Reynolds numbers.