Hydraulic Diameter Ratio Research Articles

Effect of duct length variation on solar air heater performance for smooth and D‐shaped roughened absorber plate

AbstractThe study aims to examine how duct length affects the performance of a solar air heater (SAH) with a D‐shaped ribbed absorber plate, compared to a smooth absorber plate. The optimized D‐shaped ribs from previous research investigations are utilized in the present work to explore the absorber plate's length influence on the Nusselt number. The study reveals a slight decrease in the Nusselt number as the length of the absorber plate with D‐shaped ribs is increased. The observed behavior is attributed to the diminishing capacity of air to extract heat from the heated surface within the elongated duct. Moreover, the study calculates the pressure drop and thermo–hydraulic performance parameter (THPP) associated with the D‐shaped ribs and formulates correlations to establish a quantitative understanding of the relationship between duct length and D‐shaped rib performance. The maximum value of THPP was found to be 1.17 within the considered range of duct height. Furthermore, correlations have been derived for the Nusselt number and friction factor in terms of duct length to hydraulic diameter ratio and Reynolds number with maximum deviations of +1.7 and +2.13, respectively. These correlations serve as valuable tool for engineers and researchers seeking to optimize the design of SAHs, enabling them to balance the benefits of D‐shaped ribs with the considerations of duct length. This research contributes to the growing body of knowledge of SAH design, offering insights into the trade‐offs and intricacies of utilizing D‐shaped ribs and adjusting duct length for improved THPP.

Wet cooling of air on plate finned tube heat exchangers

The objective of this paper is to provide the reliable calculation procedure for determining heat and mass flow rates for plate finned tube heat exchangers in dehumidification regimes that can be used easily in engineering practice. For this purpose, the experiments are conducted on two heat exchangers, and datasets for six more heat exchangers are used from literature. Comprehensive database is established with total of 637 sets of data, and it gathers 365 new measurement sets and 272 sets from open literature. The new calculation procedure predicts heat transfer rate and condensate flow rate where new methodology approach (based on the porous velocity, the ratio of characteristic surfaces and hydraulic diameter) is used. Predicted results show 5–10 % deviation for heat duty and up to 20 % for mass flow rate from experimental results, which is of importance to industrial practice.

Flow three-dimensionality of wavy elliptic cylinder: vortex shedding bifurcation

The three-dimensional flow of a wavy elliptic cylinder and its wake are investigated for wavelength (λ) to hydraulic diameter (Dm) ratio λ/Dm = 3.43, 4.58, and 6.01. The study aims to gain insights into flow separation, flow dynamics, and three-dimensional flow topologies for the selected wavelengths. Three-dimensional flow structures are visualized through time-averaged streamlines with critical point theory. The results for λ/Dm = 3.43 show spiral flow directed from the node to the saddle, counter-rotating vortices, and a wavy vortex structure. On the contrary, the flows for λ/Dm = 4.58 and 6.01 respectively undergo bifurcation in stable recirculation and vortex shedding on a half-span of the cylinder wavelength. The bifurcation results in a spiral flow toward the saddle and the other spiral flow toward the node. The bifurcations of recirculation and vortex shedding are revealed for the first time. The findings contribute to an improved understanding of the insight into intricate flow physics around wavy elliptic cylinders.

Enhancement of the Thermohydraulic Performance in a Double Passage Square Duct with the Use of Inclined Ribs of 45°: A Comparative Computational Study

The modern gas turbines need to run at very high inlet temperature to promote their power output. Thus, improvements in cooling technologies play a significant role for enhancing the gas turbine blade life. In this paper, thermohydraulic performance (THP) of a two-pass square channel with inclined ribs at 45° was scrutinized employing the k–ε realizable model with enhanced wall treatment in ANSYS Fluent. The calculations were performed for the rib pitch to height ratio (p/e) of 5-10, rib height to hydraulic diameter ratio (e/Dh) of 0.1-0.2, and Reynolds number (Re) of 20,000-40,000. Detailed analysis of the flow structure in a double passage square duct was carried out to understand the interaction of the rib and bend-induced secondary flows and its contribution to heat transfer enhancement for the rib configurations with distinct p/e and e/Dh, which was not available in any other existing numerical or experimental investigations. The results revealed that the ribs with higher e/Dh generated the stronger stream-wise secondary flows which led to the augmentation of the cooling performance with the disadvantage of pressure loss increment. The maximum THP of 26.55% was achieved with the ribbed configuration having p/e=5 and e/Dh=0.1 at Re=20,000. The new correlations were developed from the computational data to predict the normalized Nusselt number and friction factor (Nu/Nu0 and f/f0, where 0 is the correlation), taking the e/Dh and flow Re into consideration.

Experimental investigation on jet impingement heat transfer analysis in a channel flow embedded with V-shaped patterned surface

ABSTRACT Heating and cooling systems benefit from jet impingement as it increases efficiency while reducing operating costs. The combined methodology, integrating jet impingement and passive heat transfer through the use of roughened surfaces, offers significant potential for improving heat transfer. This research presents the results of an experimental study on a channel flow commonly used for air heating, known as a solar air heater (SAH), with impinging air on the heated surface. The surface is embedded with V-shaped ribs as turbulence promoters, and it receives a continuous heat flow of 1,000 W/m2. Various design combinations were tested experimentally, including streamwise pitch ratio X/Dh = 0.866, spanwise pitch ratio Y/Dh = 0.866, jet diameter to hydraulic diameter ratio Dj/Dh = 0.065, and an angle of attack (α) ranging from 45° to 90°. During these experiments, the Re varied from 3,500 to 18,000. The optimal improvement was observed at values of X/Dh = Y/Dh = 0.866, Dj/Dh = 0.065, and α = 60°. This paper presents novel findings demonstrating that incorporating V-shaped rib patterns in SAHs can yield Nusselt numbers up to 5.2 times higher than those in smooth duct SAHs, offering substantial potential for enhanced energy efficiency. When the entering jet impacts and flows along the ribs of the absorber, the findings suggest that the V-shaped ribs accelerate the flow, resulting in enhanced heat transfer. All datasets were also analyzed for their thermo-hydraulic performance, with the maximum value recorded as 3.301 within the constraint range used in this analysis.

Assessment of cooling performance of mini/micro-channel stacked double layer heat sink

A study is accomplished here to investigate the performance of a novel double-layer mini/micro-channel stacked heat sink (DL-MMCHSHS). The hydraulic diameter and aspect ratio of mini-channel heat sink are 0.12 cm and 3, respectively. The width of the fins between two channels is 0.04 cm. The total width of two runners is the same, and its width is 1.44 cm. The heat dissipation potential of this new type of heat sink is investigated. It was observed that the pressure loss is reduced by using the DL-MMCHSHS instead of a single-layer micro-channel heat sink (SL-MCHHS). The reduction in the pressure loss reduces the pumping power, which leads to decrease in operating costs of the system. For the total flow rate of 345.69 cm3/min, the pressure loss reduces about 35.41% by using the DL-MMCHSHS instead of a SL-MCHHS. The uniformity index in the range of 0.077 to 0.192 is achieved in this study. For the total flow rate of 346.64 cm3/min, the uniformity index enhances about 60% with boosting the flow rate ratio from 0.5 to 3.5. For the total flow rate of 1039.26 cm3/min, the thermal resistance increases about 76.22% as the flow rate ratio increases from 0.5 to 3.5.

Center-to-Center Distance’s Effect between Vertical Square Tubes of a Horizontal Array on Natural Convection Heat Transfer

An experimental study on natural convection heat transfer from the outer surface of a horizontal array of vertical square tubes in the air is investigated. The array consists of three vertical square tubes at equally different center-to-center distances. Each tube has a square cross-section with a side length of 2.00 cm, 100 cm length, and is filled with sand. Each tube is heated by inserting an internal heating element with a constant heat flux at the center. Five center-to-center separation distance to hydraulic diameter ratios (S/D) are used at different heat flux ranges of 70–360 W/m2. Results show that at small S/D, the Nusselt number of any tube in the array is lower than that of the single tube up to a specific S/D and then increases as the ratio increases. Empirical correlations are obtained for each tube in the array at different S/D using the modified Rayleigh numbers only. General correlations using S/D as a parameter are obtained for each tube, and an overall general correlation using both S/D and the tube number (n) as parameters is obtained. The difference between the predicted and experimental Nusselt numbers is in the reasonable range even at high Rayleigh numbers.

THERMOHYDRAULIC PERFORMANCE ANALYSIS OF A SOLAR AIR HEATER DESIGN EMPLOYING WAVY CORRUGATED PLATES AND IMPINGING AIR JET ARRAY

This work presents numerical investigation of the thermohydraulic performance of a wavy corrugated solar air heater design employing impinging circular air jet array. Geometrical parameters such as streamwise pitch to hydraulic diameter ratio (0.45 ≤ X/Dh ≤ 1.80), spanwise pitch to hydraulic diameter ratio (0.45 ≤ Y/Dh ≤ 1.80), jet diameter to hydraulic diameter ratio (0.06 ≤ D/Dh ≤ 0.18), duct height (0.021 m ≤ h ≤ 0.046 m), length of the duct (0.75 m ≤ L ≤ 2.25 m), and flow parameter, i.e., Reynolds number, (2000 ≤ Re ≤ 26,000) are considered for investigation. Output parameters, i.e., net power and least required outlet air temperature (≥ 40°C), are introduced. Numerical investigation presents the best thermohydraulic performance at X/Dh = 0.45, Y/Dh = 0.90, D/Dh = 0.06, and h = 0.021 m due to significant jet strength and interaction. Higher value of the Reynolds number, i.e., 18,000, is recommended to obtain effective air jet strength and high thermohydraulic performance for the present design. Moreover, the exiting air causes bending of the impinging air jets that lowers thermohydraulic performance. The maximum angle of jet deflection, 0 = 66°, is obtained at Re = 2000 and L = 2.25 m. Hence, the use of shorter collector length with impinging air jet array is strongly recommended.

Fluid flow and heat transfer characteristics of artificially roughened evacuated tube collector

An artificially roughened evacuated tube collector with periodic circular ribs was studied for heat transfer and flow characteristics. The current study was conducted in ANSYS Fluent 16.2 to analyze influence of fixed pitch to wire diameter ratio of 12 and wire diameter to hydraulic diameter ratio of 0.0643 on frictional factor and Nusselt number. RNG k-ε with enhanced wall treatment turbulence model was used for Reynolds number range of 2500–8000. For identical flow conditions, performance comparison of evacuated tube collectors having roughened and smooth absorber was undertaken. Reynolds number increase resulted in enhancement of Nusselt number and reduction in frictional factor. In comparison to smooth evacuated tube collector, augmentation of order 2.51 and 1.66 times was achieved in frictional factor and Nusselt number respectively. Across the entire Reynolds number range studied, thermohydraluic performance factor value ranged between 1.16 and 1.23.

Flow characteristics in non-conventional combustion chamber configurations

Flow characteristics in combustion chambers of several non-circular cross-sectional configurations were numerically investigated. Different cross-sectional configurations, including annular and multi-sector annulus configurations with different fillets, were proposed, examined, and compared to the conventional circular arrangement. A numerical model was established and validated against other findings. The model was then used to study the effects of different design parameters, including convex and fillet radii, as well as some operational parameters, such as the angular velocity on the total pressure drop and the secondary flow characteristics like the swirl number, secondary vortex intensity, and helicity. The results revealed the optimum number of annulus sectors to be six, at which the highest hydraulic diameter was achieved for the same cross-sectional area. Optimum values of the inner and outer corner fillets were also obtained. Several valve arrangements were also investigated to achieve the highest possible valve area ratio for the proposed configurations. One of the proposed configurations, which incorporates six annular chambers with filleted corners with a hydraulic diameter ratio of 0.991 compared to the conventional circular design, denoted AFC4, achieved a more compact design while maintaining very comparable flow characteristics to those of the circular design. Although it needs more thorough investigations, it could present a viable alternative design option for the combustion chambers.

Sustainability and Cost Effectiveness Analysis of Staggered Jet Impingement on Solar Thermal Collector

The sustainability index, waste energy ratio and improvement potential of a staggered air jet impingement on the staggered spherical protrusions of a roughened absorber plate were derived for the present study to evaluate exergy losses and irreversibility in the system. The experimental analysis was carried out for selected parameters: relative streamwise pitch, relative spanwise pitch and relative jet diameter to hydraulic diameter ratio. The flow Reynolds number ranged from 4000–18,000. The augmentation in Nusselt number and friction factor compared to a smooth surface was 4.9 and 12.4 times, respectively. The statistical correlation developed determined the maximum thermohydraulic performance parameter and exergetic efficiency be 3.02 and 3.87%, respectively. The magnitude of the sustainability index, waste energy ratio and improvement potential was found to be 1.0347, 0.962 and 10.84, respectively, for the entire range of tested parameters. A cost analysis was also performed to evaluate the cost-effectiveness of the solar thermal system with and without turbulent promoters.

A numerical study of microtube geometry effect on flow boiling using the volume of fluid method

Other than pursuing high heat flux, pressure drop also plays a key role in microtube applications as the power and efficiency of a pump are determined in terms of pressure drop. In the literature, only a few studies focus on how tube geometry influences pressure drop. This study numerically investigates the geometry effect on microtube flow boiling and pressure drop using the volume of fluid (VOF) method. Three types of geometries are used in this study: divergent, normal, and convergent microtubes with different lengths and hydraulic diameter ratios. The transient developments of vapor formation, heat flux, and pressure drops are presented within 200 ms. It is shown that the total pressure drop in the divergent microtube is the lowest. The heat flux is decreased as the massive vapor bubbles clog the tube. However, the shortened divergent microtube can increase the heat flux up to 38713.1 W/m2 and reduce the pressure drop down to 214.24 Pa, compared to the normal one under certain flow conditions, while the tube-clogging effect is not dominant. A larger inflow mass flux can delay the vapor bubble formation on the heated wall, which is beneficial to prevent the tube clogging and leads to a higher heat flux. On the other hand, higher heat flux can be achieved in the convergent microtube due to the increasing velocity inside the tube, although it induces the highest total pressure drop. This study provides a better fundamental understanding of the microtube geometry effect on flow boiling and relevant physical insight for the future design of microscale heat transfer applications.

Systematical numerical investigations on heat transfer performance of latticework channel

The heat exchanger with latticework channel is designed, optimized and fabricated for the first time to tackle high demands in fields of aeronautics, electronic device, etc. In this article, systematical numerical investigations on conjugate heat transfer are carried out for the latticework channel with the Reynolds number ranging from 100 to 1000. Due to the characteristics of the structure, the longitudinal vortex can be generated and maintained naturally without additional vortex generators, hence continuous enhanced heat transfer can be obtained. The Nusselt number can be enhanced by 3–10 times compared with that of the straight channel. By fixing the hydraulic diameter and channel aspect ratio, and adjusting the rib width, a heat transfer improvement of 28% to 69% can be achieved. Fixing the hydraulic diameter and rib width and increasing the channel aspect ratio at Re = 1000 can achieve the heat transfer improvement of 38%–70%. While in the case that the hydraulic diameter is flexible and the rib height is reduced, a heat transfer improvement of >153% can be achieved. Moreover, clear guidelines are provided on designing heat exchangers by using the latticework channel.

Numerical Investigation of Pressure Loss and Heat Transfer Characteristics in 180-Degree Bends

The pressure loss and heat transfer characteristics of single-phase water flows and non-boiling air-water flows through a slender 180° return bend with a rectangular cross-section are investigated. Such slender ducts appear in various applications. In comparison to abrupt return bends the unsymmetrical flow field affects a bigger surface. Therefore, there is an interest to quantify the impact of such curvature effects. For the pressure loss consideration, the curvature ratios are 19.9, 40, and 58.1, whereas for heat transfer, the focus is on the latter, the hydraulic diameter and aspect ratio are 3.9 mm and 1.067, respectively. The numerical results are compared to published experimental ones. From the numerical modeling results, where three different approaches of turbulence modeling were tested, it is confirmed that above a curvature ratio of 90 it seems reasonable to neglect curvature effects on pressure drop. The validated conjugate heat transfer models are utilized to develop local and global relationships for the Nusselt number ratios between the convex and concave surface. The two-phase flow modeling is realized via a homogenous flow model approximation. The results show that the modeling provides reasonable results and that the impact of curvature on such flows can be relevant.

Effect of Rotation on Heat Transfer in AR = 2:1 and AR = 4:1 Channels Connected by a Series of Crossover Jets

Abstract Detailed heat transfer measurements using transient liquid crystal thermography were performed on a novel cooling design covering the mid-chord and trailing edge region of a typical gas turbine blade under stationary and rotating conditions. The test section comprised two channels with aspect ratio (AR) of 2:1 (mid-chord) and 4:1 (trailing edge), where the coolant was fed into the AR = 2:1 channel from the root. Rib turbulators with a pitch-to-rib height ratio (p/e) of 10 and rib height-to-channel hydraulic diameter ratio (e/Dh) of 0.075 were placed in the AR = 2:1 channel at an angle of 60 deg relative to the direction of flow. The coolant after entering this section was routed to the AR = 4:1 section through a set of crossover jets. The purpose of the crossover jets was to induce sideways impingement onto the pin fins that were placed in the 4:1 section to enhance heat transfer. The 4:1 section had a realistic trapezoidal shape that mimics the trailing edge of an actual gas turbine blade. The pin fins were arranged in a staggered array with a center-to-center spacing of 2.5 times the pin diameter in both spanwise and streamwise directions. The trailing edge section consisted of both radial and cutback exit holes for flow exit. Experiments were performed for the Reynolds number (Redh(AR=2:1)) of 20,000 at Rotation numbers (Rodh(AR=2:1)) of 0, 0.1, and 0.14. The channel-averaged heat transfer coefficient on trailing side was ∼28% (AR = 2:1) and ∼7.6% (AR = 4:1) higher than the leading side for Rotation number (Ro) of 0.1. It is shown that the combination of crossover jets and pin fins can be an effective method for cooling wedge-shaped trailing edge channels over axial cooling flow designs.

Experimental studies of turbulent pulsating flow and heat transfer in a serpentine channel with winglike turbulators

Particle Image Velocimetry and Infrared Thermography measurements are applied to explore the effect of quasi-triangular pulsatile flow inlet on active thermal-fluidic performance in a serpentine channel with innovative winglike turbulators. The novel turbulators are mounted in-line on two sidewalls of the channel with the attack angle (α), thickness to chord line ratio (t/C), and pitch to the channel hydraulic diameter ratio (Pi/DH) respectively fixed at 20°, 0.2, and 0.7. The Strouhal number (St) varies from 0 to 0.67 at a fixed Reynolds number (Re) of 10,000. Relative to the steady-state case (St = 0), the flow pulsation not only strengthens the Fujiwhara co-rotating vortices but also eliminates the corner vortex streams, leading to the overall Nusselt number (Nu¯/Nuo) enhancement by 13%. Moreover, both the Nu¯/Nuo and friction factor (f¯/fo) reveal the tilde-like trends with St, resulting in the highest Nu¯/Nuo of 5.2 and thermal performance factor of 1.39 at St = 0.67, which are higher than the previous best results for 10≤f¯/fo≤100 in the Nu¯/Nuo-f¯/fo diagram. Finally, by combing the present data with our previous steady state results, composite correlations of Nu¯/Nuo and f¯/fo versus Re, α, t/C, Pi/DH, and St are proposed.

Numerical investigation of compressibility effects on friction factor in rectangular microchannels

This study numerically investigated the effect of gas compressibility on the friction factor in rectangular microchannels. The numerical model adopted in the study was validated against experimental data obtained by testing rectangular microchannels with a hydraulic diameter of 295 μm. The numerical model was used to evaluate the Reynolds number at which the compressibility effects on the friction factor became significant by analyzing the role of both hydraulic diameter and aspect ratio of the microchannel. To this end, three values of the hydraulic diameter (100, 295, and 500 μm) and five different aspect ratios (from 0.25 to 1) were investigated. The results showed that compressibility effects became increasingly stronger by reducing the hydraulic diameter and that they led to an increase in the average friction factor. For smaller microchannels, the Reynolds numbers at which the compressibility effects became significant tended to reduce and were in the laminar regime. The gas compressibility could not be ignored when friction factors needed to be accurately determined. Moreover, for narrow microchannels (low aspect ratio), compressibility effects became important for higher values of the Reynolds number than those observed for nearly squared microchannels.

Effects of baffles and vortex generators on cooling performance of a gas turbine combustion chamber: Numerical assessment

In this work, the effect of using baffles and vortex generators on the cooling performance of a gas turbine combustion chamber is numerically investigated. The RNG k-ɛ model is utilized in order to simulate the flow turbulence. The effects of baffles and vortex generator pitch, hydraulic diameter and bypass ratio on the Nusselt number and friction coefficient at different Reynolds number are studied. The cooling performance of applying baffles and vortex generators are also evaluated and compared based on the Performance Evaluation Criterion (PEC). The numerical results are verified with available experimental data. The results show that the pitch increment results in the decrement of the Nusselt number and the wall friction coefficient due to the lower vortices generation and the turbulence kinetic energy dissipation. Also, the Nusselt number is increased by increasing both the bypass ratio and the hydraulic diameter. It is also found that vortex generators and baffles improve the cooling performance of the gas turbine combustion chamber by 18.8% and 26% respectively through decreasing the external wall temperature.

Numerical study of heat transfer and flow structure over channel surfaces featuring miniature V rib-dimples with various configurations

In order to enhance the heat transfer performance of the internal cooling for gas turbine blades, miniature V rib-dimple hybrid structures on cooling channel surfaces were studied numerically at Re = 50,500, and the effects of parameters of rib heights, dimple depths, rib pitches and rib-to-dimple pitches were detailly investigated on the turbulent flow structure, pressure loss and heat transfer characteristics. The Abe Kondoh Nagano (AKN) k−ε turbulence model was utilized for the numerical simulation work. The results showed that as the rib height and dimple depth increase, the globally averaged heat transfer augmentation increases significantly at first, and then becomes stable. And the maximum heat transfer enhancement is reached when the rib height-to-channel hydraulic diameter ratio is 0.075 and the dimple depth-to-diameter ratio is 0.2. Besides, the more densely fabricated hybrid structures induce higher heat transfer, while a relatively larger spacing between the rib and the downstream dimple also obtains higher heat transfer. Within the parameters of this study, the total heat transfer enhancement of the hybrid structures can be up to 95.3% and 43.2% respectively higher than that of the dimple-only and the rib-only structures. The optimal hybrid structure with the overall thermal performance factor of 1.73, with e/Dh=0.075, δ/d = 0.2, P1/e = 7.2 and L/P1 = 0.83, has a total Nusselt number ratio of 3.9 and a friction ratio of 11.6. It is evident that with the miniature V rib-dimple hybrid structure, the turbulent kinetic energy of the flow passing over the V ribs is enhanced, thereby reducing the recirculation zone in the front of the dimples. Additionally, the vortex flow induced by the V rib strongly interacts with the downstream dimpled wall which significantly augments the overall momentum and heat transport of the near-wall flow field, and contributes to enhanced and more uniformly distributed heat transfer on the channel surface.

Canopy-to-canopy liquid cooling for the thermal management of lithium-ion batteries, a constructal approach

With the growing interest on electric vehicles comes the question of the thermal management of their battery pack. In this work, we propose a thermally efficient solution consisting in inserting between the cells a liquid cooling system based on constructal canopy-to-canopy architectures. In such systems, the cooling fluid is driven from a trunk channel to perpendicular branches that make the tree canopy. An opposite tree collects the liquid in such a way that the two trees match canopy-to-canopy.The configuration of the cooling solution is predicted following the constructal methodology, leading to the choice of the hydraulic diameter ratios. We show that such configurations allow extracting most of the non-uniformly generated heat by the battery cell during the discharging phase, while using a small mass flow rate.