Pressure Loss Penalty Research Articles

Impact of delta winglet vortex generators on the performance of a novel fin-tube surfaces with two rows of tubes in different diameters

To achieve heat transfer enhancement and lower pressure loss penalty, even pressure loss reduction, two novel fin-tube surface with two rows of tubes in different diameters are presented in this paper. Numerical simulation results show that the fin-tube surface with first row tube in smaller size and second row tube in larger size can lead to an increase of heat transfer and decrease of pressure drop in comparison with the traditional fin-tube surface with two rows of tubes in the same size. Based on this understanding, delta winglet pairs are punched out only from the larger fin area around the first transverse row of tubes in smaller size in the novel fin-tube surfaces. Delta winglet pairs used as longitudinal vortex generator are arranged either in “common flow up” or “common flow down” configurations. Numerical simulation results show that delta winglet pairs can bring about a further heat transfer enhancement and pressure drop decrease through the careful arrangement of the location, size and attack angle of delta winglet pairs either in “common flow up” or “common flow down” configurations. The traditional knowledge of heat transfer enhancement with necessary pressure drop increase is challenged by the present conclusion. The present work will be helpful to develop more compact, higher heat transfer efficiency, lower fan power and quieter heat exchanger of refrigeration and air condition system.

Optimal Shape Design of Compact Heat Exchangers Based on Adjoint Analysis of Momentum and Heat Transfer

An adjoint-based shape optimization method of heat exchangers, which takes into account the heat transfer performance with the pressure loss penalty, is proposed, and its effectiveness is examined through a series of numerical simulation. Undulated heat transfer surface is optimized under an isothermal heated condition based on the variational method with the first derivative of the cost function, which is determined by an adjoint analysis of momentum and heat transfer. When applied to a modeled heat-exchanger passage with a pair of oblique wavy walls, the present optimization method refines the duct shape so as to enhance the heat transfer while suppressing the flow separation. It is shown that the j/f factor is further increased by 4% from the best value of the initial obliquely wavy duct. The effects of the initial wave amplitude upon the shape evolution process are also investigated.

Numerical Investigation for Net Enhancement in Thermal-Hydraulic Performance of Compact Fin-Tube Heat Exchangers with Vortex Generators

Systematic numerical simulations are carried out for a steady-state, laminar and constant-property air flow through a passage of a compact fin-tube heat exchanger specified with a constant wall temperature distribution and a uniform inlet velocity profile. A pair of delta winglet-type vortex generators (VGs) is punched out of the plain fin surface near the tube as a heat transfer enhancement device. A variety of VG configurations are investigated and their effects on the thermal-hydraulic (heat transfer) performance of the compact fin-tube heat exchanger are presented in terms of the Colburn factor j and the friction factor f for a specific Reynolds number (Re=400). In addition, the highest net enhancement in the thermal-hydraulic performance, defined as the ratio of the heat transfer enhancement to the pressure loss penalty, is sought. It is found that a moderate degree of the upstream shifting and the spanwise shifting of VG toward the tube contribute to the net enhancement. Nearly 9% of the net enhancement is achieved with an optimum configuration investigated presently.

Laminar flow and heat transfer in a periodic trapezoidal channel with semi-circular cross-section

Computational fluid dynamics (CFD) has been used to study fully developed laminar flow and heat transfer behaviour in periodic trapezoidal channels with a semi-circular cross-section. The trapezoidal elements are characterised by their wavelength (2 L), channel diameter ( d), radius of curvature of bends ( R c), the amplitude (2 A) and the length of the straight section ( B) with results reported for Reynolds numbers ( Re) up to 400, as well as for a range of geometric configurations ( 0.525 ⩽ R c d ⩽ 1.3 , 3.6 ⩽ L d ⩽ 12 , 0.17 ⩽ B L ⩽ 1 , 0.125 ⩽ A L ⩽ 1 ) at Re = 200. This generic geometry takes a variety of shapes with limiting forms of a regular square serpentine ( B = 2 A = L) and a zig-zag or saw-tooth ( B → 0). The flow in these channels is characterised by the formation of Dean vortices following each bend. As the Reynolds number is increased, stronger vortical flow patterns emerge and these vortices lead to efficient fluid mixing and high rates of heat transfer. Constant wall heat flux (H2), constant axial heat flux with peripherally constant temperature (H1) and constant wall temperature (T) boundary conditions are examined for a fluid with a Prandtl number of 6.13. Higher rates of heat transfer with relatively small pressure loss penalty are found relative to fully developed flow in a straight pipe, with heat transfer enhancements of up to four at the highest Reynolds number. In addition to presenting channel enhancements the stackability of channels on a plate is considered. The concepts of area enhancement (based solely on geometric factors) and heat transfer intensification, the product of the heat transfer enhancement and the area enhancement, are introduced and used to compare different geometrical configurations. The swept zig-zag pathway provided the greatest intensification of heat transfer in a multi-channel plate structure.

Optimal Shape Design of Compact Heat Exchanger Based on Adjoint Analysis of Momentum and Heat Transfer

An adjoint-based shape optimization method of heat exchanger, which takes into account the heat transfer performance with the pressure loss penalty, is proposed, and its effectiveness is examined through a series of numerical simulation. The undulated heat transfer surface under an isothermal heated condition is optimized based on the variational method with the first derivative of the cost function, which is determined by an adjoint analysis of momentum and heat transfer. When applied to a modeled heat-exchanger passage with a pair of oblique wavy walls, the present optimization method refines the duct shape in a phased manner so as to enhance the wall shear while suppressing the flow separation. It is shown that the j/f factor is further increased by 4% from the best value of the initial obliquely wavy duct. The effects of the wave amplitude, which is initially assumed, upon the shape evolution process are also investigated.

Assessment of compact heat exchanger consisting entirely of tubes arranged in a turbulence generating mesh

The performance of a novel liquid-air heat exchanger is experimentally evaluated and compared with that of several commercial automotive radiators having louvre fins and flat tubes. The coils were assessed using a closed loop test rig which has the capability of simulating ambient conditions of any habitable place on earth. The prototype heat exchanger was designed to generate three dimensional vortical motion using an open mesh structure, to enhance heat transfer without incurring high pressure loss penalties. When compared with respect to the available heat transfer surface area, the heat transfer capacity, and the heat transfer coefficient were increased. Also the pressure drop was well below that of the conventional radiators. However, in a direct comparison the overall the heat transfer capacity fell short of expected commercial radiator requirements.

Simultaneous heat transfer enhancement and pressure loss reduction for finned-tube bundles with the first or two transverse rows of built-in winglets

Heat transfer enhancement and pressure loss penalty caused by a single row of winglets built in the first transverse row of tubes, are compared between “common flow up” (“toe-out”) and “common flow down” (“toe-in”) winglet configurations. For the three-row tube bundle in a staggered arrangement, the “toe-in” winglet-pairs bring about 5–15% increase in heat transfer enhancement and 2–10% increase in pressure loss penalty, in comparison with fin-tube bundles without winglet. In the present authors’ previous paper [K. Torii, K.M. Kwak, K. Nishino, Heat transfer enhancement accompanying pressure-loss reduction with winglet-type vortex generators for fin-tube heat exchangers, International Journal of Heat Mass Transfer 45 (2002) 3795–3801], “toe-out” winglet-pairs built only in the first transverse row of three-row tube bundle in a staggered arrangement successfully increased heat transfer by 10–30%, and simultaneously reduced pressure loss by 34–55%, for the Reynolds number (based on two times channel height) ranging from 350 to 2100. In the present paper, the front two rows of the “toe-out” winglet-pairs were applied for a three-row tube bundle in a staggered/in-line arrangement, for comparison. Heat transfer and pressure loss in the staggered arrangement were increased by 6–15% and by 61–117%, respectively, in comparison with the case of the single transverse row of the “toe-out” winglet-pairs. The corresponding increases for an in-line arrangement with two rows of winglets were 7–9% and 3–9%, respectively.

Heat Transfer and Fluid Flow Characteristics of Recuperators with Oblique Wavy Walls

A series of direct numerical simulation in modeled counter-flow heat exchangers with oblique wavy walls is made toward optimal shape design of recuperators. The effects of oblique angles and amplitudes of the wavy walls are systematically examined, and the heat transfer and pressure loss characteristics are investigated. The flow structures are drastically modified due to the counterrotating streamwise vortices induced by the wavy walls. By using optimal oblique angles and amplitude of the wavy walls, significant heat transfer enhancement is achieved with relativelysmall pressure loss penalty. When thermal coupling of hot and cold fluid passages is considered, the averaged Nusselt number can be larger than that for isothermal heated condition. This is due to the dissimilarity between the velocity and thermal fields near the top and bottom walls. It is also found that the secondary flow and the associated thermal fields strongly depend on the Reynolds number.

UNSTEADY FLOW AND HEAT TRANSFER IN PARALLEL-PLATE HEAT EXCHANGERS WITH IN-LINE AND STAGGERED ARRAYS OF POSTS

A numerical study has been carried out to analyze the unsteady three-dimensional flow and conjugate heat transfer in a channel with in-line and staggered arrays of periodically mounted square posts. The simulations have been carried out for Reynolds numbers in the range 150–600 (based on the post width and the average velocity), and a Prandtl number of 6.99 (corresponding to water). The flow is found to be unsteady at a critical Reynolds number around 400 for the in-line case, while it is unsteady at Re = 150 (the lowest Reynolds number studied) for the staggered case. The unsteady flow shows quasi-periodic behavior with multiple frequencies (having different dominant shedding frequencies). For the in-line configuration, the heat transfer enhancement is small when the flow is steady (2.28–2.88 relative to a plane channel flow for Re ≤ 250), but it is significant when the flow is unsteady (17 times that for a plane channel flow for Re = 600). The staggered case shows significant enhancement at all Reynolds number (19–66), but with an associated pressure loss penalty (54–150 times the value for the plane channel flow). However, the thermal performance factor for the in-line configuration has lower values than the staggered configuration at all Reynolds numbers.

D231 斜め波状壁を用いた再生熱交換器の形状最適設計

Abstract: A series of numerical simulation of the flow and heat transfer in modeled counter-flow heat exchangers with oblique wavy walls is made for optimal shape design of recuperators. The effects of oblique angles and height amplitudes of the wavy walls are systematically examined, and the heat transfer and pressure loss characteristics are investigated. Strong secondary flow is generated by counter-rotating streamwise vortices, which make the flow field highly asymmetric in the spanwise direction. With the oblique angle of 50-60 degree, significant heat transfer enhancement is achieved with relatively-small pressure loss penalty, and the j/f factor becomes significantly larger than that of straight square duct. When thermal coupling of hot and cold fluid passages is considered, the heat transfer is found to be strongly dependent on the arrangement of the passages. It is also found that the j/f factor is increased with the Reynolds number in the range of the present study.

Heat transfer and pressure loss penalty for the number of tube rows of staggered finned-tube bundles with a single transverse row of winglets

The objective of this research is to investigate the heat transfer and pressure loss penalty for various numbers of transverse rows in staggered finned-tube bundles with a single transverse row of the winglet pairs beside the front row of the tube bundles. Experiments were performed for two, three, four and five rows of staggered tube bundles. The pairs of winglets were placed with a heretofore-unused orientation for the purpose of augmentation of heat transfer and reduction of pressure loss penalty. This orientation is called as “common flow up” configuration. For three rows of tubes with a single transverse row of winglets beside the front row of the tubes, the heat transfer was augmented by 30–10%, and yet the pressure loss was reduced by 55–34% with the increase of the Reynolds number (based on two times channel height) from 350 to 2100. The reduction of the pressure loss penalty for three rows of tube bundles is the largest in comparison with the other numbers of rows.

Numerical simulation for heat and fluid characteristics of square duct with discrete rib turbulators

Attachment of rib turbulators to flow passages is one of the popular means of heat transfer enhancement. Typical use of the rib turbulators is found for example in the serpentine cooling air channel for the internal cooling of the gas turbine blades. Many studies therefore have been carried out for the heat transfer in ribbed channels and various types of ribs have been tested. A standard case of full-span ribs attached perpendicularly to the flow direction has been the first target and its friction and heat transfer characteristics have been reported in the references [1–3]. Further studies were made for other cases of attaching oblique ribs and V-shaped ribs to the channel wall. In these cases, secondary flow is incurred in the channel and enhances the fluid mixing between the near wall and core regions in the duct, eventually resulting in the enhancement of local heat transfer at the channel walls [4–6]. Discrete ribs also drew attention with which heat transfer enhancement due to the incurred secondary flow is achieved paying smaller pressure loss penalty [7– 10]. In relation with the existence of the spanwise gaps, secondary flows are generated downstream their spanwise edges. Generation of the secondary flows in addition to the flow separation and reattachment makes the phenomenon quite complicated with the discrete ribs. Detailed studies of the flow structure and characteristics of the related heat transfer are required but such studies have scarcely been made so far. In the present article, some results of the three-dimensional numerical computation conducted for flow and thermal fields over two types of array of ribs attached to a channel wall will be presented. One is of fullspan ribs and the other is of discrete ribs, where both arrays are attached in a position perpendicular to the flow direction. The discussion will be given to the effects of three-dimensionality and unsteadiness of the flow and thermal fields for twofold purposes, one to test the applicability of numerical study to the type of flow under concern and another to provide hints to experimental works to be made in future.

Heat (mass) transfer in rectangular cross-sectioned two-pass channels with an inclined divider wall

Heat transfer characteristics in rectangular cross-sectioned two-pass channels with an inclined divider (inner) wall have been examined experimentally. Local heat (mass) transfer rates were measured by the naphthalene sublimation method; seven kinds of divider inclination angles were tested for three turn clearances under the Reynolds numbers of (2.0–5.0)×10 4. The influence of the inclined divider wall on the local heat transfer characteristics is discussed in detail. Then, the optimum combination of the inclination angle and the turn clearance is examined based on the trade-off between the heat transfer enhancement and pressure loss penalty, and on the improvement of uniformity in the distribution of local heat transfer rates.

AEROSOL DEPOSITION IN TURBULENT CHANNEL FLOW ON A REGULAR ARRAY OF THREE-DIMENSIONAL ROUGHNESS ELEMENTS

Understanding particle deposition onto rough surfaces is important for many engineering and environmental applications. An experimental system was designed for the study of aerosol deposition on regular arrays of uniform elements (in the form of discrete protrusions) in a turbulent ventilation duct flow using monodisperse tracer small particles, in the range 0.7– 7.1 μ m . The Reynolds number for the test conditions was 44,000 in the 150 mm square duct. The roughness elements were arranged at two different orientations with respect to the airflow direction and the aerosol deposition velocity and pressure drop were measured for both orientations. Compared to earlier measurements in the same duct system involving smooth or ribbed surfaces, a significant increase in deposition velocity onto the regular roughness elements is observed. In addition, the associated pressure loss penalty is lower than in the presence of the roughness elements than in the presence of the ribbed surfaces. This may be attributable to the small dimensionless roughness height of the elements, which results only in a moderate distortion of the flow structure near the surfaces.

Heat-transfer augmentation by longitudinal vortices rows

Longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty. The objective of this experimental work is to investigate the influence of arrays of longitudinal vortices generated by half-delta wings on the local and average heat transfer and fluid flow of an otherwise laminar boundary layer. Heat transfer experiments and hot-wire velocity measurements were conducted in order to clarify the mechanisms of heat transfer augmentation. Experiments with arrays of co-rotating and counter-rotating longitudinal vortices indicated that both laminar and turbulent effects play a key role in enhancing heat transfer. Counter-rotating longitudinal vortices tend to merge in the common flow up region between generators; in this case, smaller distances between generators with the common flow down and large angles of attack were found to enhance the heat transfer. Arrays of co-rotating longitudinal vortices show lower vortex merging; arrays of vortices produced by half-delta wings with higher angles of attack significantly distort the flow field and produce a noticeable growth of near-wall turbulence intensity. An overall performance analysis indicated that arrays of counter-rotating longitudinal vortices are more suitable for heat transfer enhancement than arrays of co-rotating vortices.