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Abstract: Aiming at achieving a more comprehensive understanding of influence factors on thermal-hydraulic characteristics for high-pressure-direction type steel offset strip fins, this paper constructed a single fin core assembly test rig with lubricant oil as work media at low Reynolds number. Six fin schemes were experimentally investigated only varying in fin height and fin wavelength due to mould restrictions. Twenty fin schemes changing in fin width, fin angle, fin thickness and staggered fin wavelength were simulated for supplement, and the geometrical influences in fin performance were put forward. Both Taguchi method and uniform design were introduced to study the contribution of each geometrical factor to fin global thermal-hydraulic performance, verifying that fin wavelength and fin height have the most significant contributions. Based on signal to noise ratio distributions, fin geometries were optimized for 16% elevation of global performance at Reynolds number of 430, then the ‘cost-based fin performance’ was put forward and compared quantitatively, proving that above optimum fin scheme is also economical. Finally, mould wear influence in fin performance was tested in different wear periods of a vacuum heat-treated Cr12MoV mould pair, then three stages of initial wear, stable wear, and overdrive wear were defined based on pressing times. The influences were analyzed and regulations were deduced relating to mould pressing times and oil Reynolds number. The suggested lifetime for this type of mould pair is 1050 thousand pressing times. The paper intends for fin performance evaluation, optimization, and pressing mould improvement. [Copyright &y& Elsevier]

In this paper, thermohydraulic characteristics and enhanced heat transfer mechanism in water–cooling oil heat exchanger with various shapes and attack angles of vortex generators (VGs) and spacing of double VGs are numerically investigated at different Reynolds number (Re) and oil–water mediums. A CFD-aided method, conjugated heat transfer model, is performed to elaborate the correlation between heat transfer and flow characteristics. The quantitative and qualitative results manifest that the longitudinal and secondary vortices, induced by VGs, generate synergistically the tapered and elliptical enhanced heat transfer regions on the heat exchange fins, but the inhibiting effect still exists between both, particularly in high attack angle or oil medium. Elliptical winglet VG with 30° in oil medium with Re = 349 and 15° in water medium with Re = 2726 gains the maximum thermal performance factor, reaching 1.69 and 1.28, respectively, compared without VGs. Furthermore, the appropriate configuration of two core heat transfer regions is a cost-optimal method to obtain in-depth synergistic heat transfer. The spacing of 4 mm shows the best synergistic heat transfer, and the maximum heat flux of its was elevated to 2.2% and 8.6% more than that of single VG in oil and water medium, respectively. [ABSTRACT FROM AUTHOR]

The evaporation processes at low temperatures are essential for the factories and thermal systems as the heat pumps by absorption. This paper shows the design, building and characterization about an evaporator of 2 kW with a steam generation at 323.15 K. Its geometry consists in three nested helical coils with different diameters. Every coil is connected in series and the heat interchange is given by means of pool boiling. The heat transfer coefficients are indicated considering different operating conditions. The sizing method was validated taking into account the theoretical and experimental overall heat transfer coefficient of 659.8 W/m 2 K. The evaporator efficiency was up to 83% taking into account a steam of 7.1 × 10 −4 kg/s with a heat source at 334.35 K and 0.11 kg/s. [ABSTRACT FROM AUTHOR]

Longitudinal vortex generators (LVGs) are among the most popular actuators for generating longitudinal vortices for the fin-side heat transfer enhancement of heat exchangers. In order to obtain a better heat transfer performance, researchers always try to punch lots of vortex generators out of the fin surface. However, the increasing number of LVGs does not necessarily increase the heat transfer performance due to the interaction of longitudinal vortices meeting in the flow channel. The interaction of longitudinal vortices not only affects the intensity of longitudinal vortices but also the heat transfer enhancement. In this paper, the intensity of longitudinal vortices generated by LVGs is quantitatively studied in the fin-side channel of flat-tube-bank-fin heat exchanger by using the parameter Se for secondary flow intensity. The effect of transverse pitches of LVGs on the interaction of longitudinal vortices is studied in detail. What is also studied includes the effects of interaction of longitudinal vortices on the intensity of longitudinal vortices and the characteristics of periodically developed fluid flow and heat transfer. The results present that the transverse pitches of LVGs are related to the interaction between longitudinal vortices. Also, the intensity of longitudinal vortices and the heat transfer performances of longitudinal vortices are obviously affected by the interaction of longitudinal vortices. Optimum transverse pitch of LVGs is necessary for the best heat transfer performance. Meanwhile, an undesired transverse pitch of LVGs should be avoided. [ABSTRACT FROM AUTHOR]

Heat transfer, pressure drop and effectiveness in a double pipe heat exchanger made of corrugated outer and inner tubes have been experimentally investigated in this paper. Both of the inner and outer tubes were corrugated by means of a special machine. New various arrangements of convex and concave corrugated tube were investigated. Heat transfer coefficient was determined using Wilson plots. Hot water (inner tube) and cold water (outer tube) inlet temperatures were maintained at around 40 °C and 8 °C respectively. Experiments were performed over the Reynolds number range of 3500–18,000, based on the hydraulic diameter of the annular space between the two tubes. Hot water Reynolds number was kept constant at around 5500. Findings indicated that the outer tube corrugations and arrangement type of corrugated tubes have significant effect on thermal and frictional characteristics. Maximum effectiveness was obtained for heat exchanger made of concave corrugated outer tube and convex corrugated inner tube. [ABSTRACT FROM AUTHOR]

In this paper, a new design of heat exchanger is proposed and designed to recover exergy from exhaust of a diesel engine. Twenty vortex generators with optimum dimensions and angle of attack are located in the exhaust to reach more exergy recovery as well as low pressure drop. The experiments are done in five engine loads (0, 20, 40, 60 and 80% of full load) and four water mass flow rates (20, 30, 40 and 50 g/s). After the exergy analysis, results of irreversibility, recovered heat and exergy amount, pressure drop, etc. are compared in different engine loads and speeds. Also, the results of Nusselt number, friction factor and recovered exergy from this new heat exchanger design are compared with simple double pipe heat exchangers. [ABSTRACT FROM AUTHOR]

The present paper explores the possibility of using wire-loop structures on the active plate of a parallel plate channel for efficient heat transfer augmentation. For this purpose, numerical simulations were carried out for periodically fully-developed turbulent flow in typical repeating modules using the realizable k – ε model. Experiments were also conducted in order to validate some of the results obtained from numerical simulations. For both studies, the Reynolds number was varied from 2000 to 20,000. The effect of three different loop-densities on fluid flow and heat transfer characteristics were investigated when wire-loops were placed perpendicular to the main flow direction, whereas the effect of loop-orientations on these parameters were studied for a fixed loop density of 2270 loops/m 2 . While for all investigated cases, substantial heat transfer augmentation was observed with wire-loop structures as compared to the empty parallel plate channel under the condition of identical pressure gradient, the thermal-hydraulic performance improved significantly with the increase in loop-density. The maximum attainable loop-density, however, was found to strongly depend on the loop-orientation owing mainly to the geometric as well as manufacturing constraints. It was also observed that loops, oriented diagonally to the main flow direction offer the best performance, although it is less sensitive to loop-orientation as compared to the loop-density. [ABSTRACT FROM AUTHOR]

The work aims to improve the accuracy of an analytical heat exchanger dynamic model published recently in the International Journal of Heat and Mass Transfer. As shown in that paper, an analytical model to predict the dynamic performances is useful in some specific problems such as the heat exchanger network design. The proposed model illustrated considerable progress compared with the original one in the dynamic performance prediction aspect through a case problem. In which the accuracy of the time response prediction was improved more than 80%. In addition to increasing the accuracy, the new model also allows reaching the outlet temperature in the time domain with an analytical solution and free of convergence issues. As is the original model, the improved one is still capable of dealing with simultaneous inlet variations and various inlet temperature change forms, such as step, exponential or ramp functions. [ABSTRACT FROM AUTHOR]

Abstract: The work presented in this paper reports the quantitative study on the influence of the statistical moments of probability density function, i.e., mean, standard deviation, skew and kurtosis, for an air flow maldistribution profile on the thermal performance of a fin-tube heat exchanger. The effects of the geometrical parameters of the heat exchanger, i.e., tube diameter, fin pitch, row pitch, tube pitch, number of rows and fin surface pattern, are also investigated. The thermal performance is found to be noticeably affected by mean, standard deviation and skew but not the kurtosis. In addition, the interaction between the external and internal thermal resistances has a significant effect. All the geometrical parameters examined, except for the number of rows and fin pattern, have either a weak or an insignificant effect on the thermal performance degradation factor. Physical reasoning has been provided to explain the trends of the degradation with respect to the moments and geometrical parameters. From these trends, a new set of correlation equations is proposed to predict the degradation effect of the flow maldistribution on wavy fins. The correlation agrees well with experimental data within ±10%. [Copyright &y& Elsevier]

Abstract: The work presented in this paper investigates the degradation effect of flow maldistribution on the thermal and hydraulic performance of a heat exchanger. A new mathematical model is derived based on Taylor series expansion to describe the contribution of each of the four statistical moments of distribution on the degradation problem. It is found that the first two moments, i.e. mean and standard deviation, have the highest effect on the performance degradation. Subsequent higher moments will give declining degradation effects until the fourth moment, kurtosis, which has no significant effect. Maldistribution with low standard deviation and high positive skew will give low thermal deteriorations, though a distribution with negative skew is preferred for low hydraulic performance losses. The design of the heat exchanger could be modified to give these favourable moments which would minimize the degradation effects of the maldistribution. Consequently, any effort to modify the flow distribution profile should be focused on optimizing the first three moments. [ABSTRACT FROM AUTHOR]

Abstract: This article analyzes parallel flow microchannel heat exchangers with non-adiabatic thermal condition imposed on the wall separating the fluids and the ambient. For purposes of analysis, a thermal model comprising of two governing equations, one for each of the fluids, is developed and analytically solved to obtain equations for calculating the axial temperature and thereby the effectiveness of both fluids. Provision is provided in these equations for the temperature of the ambient interacting with the hot fluid to be different from that associated with the cold fluid. Analytical equations for determining the heat transfer between the individual fluids and the respective ambient as well as that between the fluids are also developed in this paper. The temperatures/effectiveness of the fluids depend on parameters such as NTU, fluid heat capacities, thermal resistance between the individual fluid and the respective ambient and ambient temperatures. In addition, under certain operating conditions the phenomenon of temperature-cross is observed. Depending on the ambient temperatures, increase in NTU will continuously improve the effectiveness of one the fluids; the effectiveness of the other fluid over the same NTU range increases before peaking and subsequent decrease. Equations for determining the NTU at which this peak value in effectiveness occurs are also provided. An unbalanced flow microchannel heat exchanger with the hot fluid having the lowest heat capacity has better effectiveness than when the cold fluid has the lowest heat capacity. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a method for the detection of fouling in a cross-flow heat exchanger. A numerical model is used to generate data when the heat exchanger is clean and corresponding data when fouling occurs. In a first step, the model is used to generate a long time series by simulating a clean heat exchanger. This allows the determination of a neural network model of the heat exchanger. Then, hundred sets of data are generated by simulating a fouled heat exchanger and it is checked that the simple Cusum test can be used to detect fouling without any false alarm, whatever the reference time series is. [Copyright &y& Elsevier]

Abstract: The present paper analyzes the entropy production rate induced by longitudinal vortex generators punched on the fins of a compact heat exchanger. The flow is assumed to be three-dimensional, unsteady and laminar. The entropy production rate due to heat fluxes and drag forces is studied by means of finite volume methodology. The heat conduction in the fin is also taken into account and hence the induced thermal entropy production is also evaluated in the fin. The effects of the fin efficiency (factor Fi) and of the angle β of the vortex generator are evaluated for the volumetric entropy production rate in the fin and in the flow field. The integration over the whole domain enables to define the entropy production number . The relevance of this exchanger global criterion is illustrated for a specific technical requirement. The results show that the volumetric entropy production rate is predominant in the strong flow gradient zones. These zones correspond to the outer surface of the longitudinal vortex and to the thinning zone of the boundary layer (down-wash zone). This entropy production increases with Fi and β. In the fin the entropy production decreases with Fi. It is shown that the Bejan number (Be) is close to unity, which means that the global entropy production rate due to heat transfer is predominant in the flow field. The effectiveness of the heat exchanger increases with Fi and β, which corresponds to an increase of the Number of Transfer Unit NTU, but a decrease of . [Copyright &y& Elsevier]

The effect of non-uniform flow on thermal performance of heat exchanger was analyzed in this paper. The quantitative relationships between velocity deviation factor and heat transfer coefficient, heat transfer capacity, entransy-dissipation-based thermal resistance, were obtained, respectively. Based on these relationships, an evaluation plot was proposed to evaluate the effect of flow maldistribution on thermal performance. The plot was divided into three zones according to various degrees of deterioration. In Zone Ⅰ (stable zone), the variations of amplitude of heat transfer characteristics is negligible with the increase of the velocity deviation factor. In Zone Ⅱ (deterioration zone), the increasing velocity deviation factor results in thermal performance reducing in a modest rate. In Zone Ⅲ (rapid deterioration zone), thermal performance deteriorates rapidly with the increase of velocity deviation factor. The predicted results of the evaluation plot were compared with abundant simulations and experiments from references, which cover cases of variable velocity deviation factors, different flow distribution profiles and various types of heat exchangers. The excellent agreement of the comparisons has proved that the plot can be successfully applied to rapidly evaluate the effect of non-uniform flow on thermal performance of heat exchanger with high accuracy. Meanwhile, traditional researches on thermal performance of heat exchanger with uniform flow can be easily translated into that of non-uniform flow by this plot method. • Quantitative relationship between fluid non-uniformity and thermal performance is obtained. • Evaluation plot is established to rapidly evaluate the effect of fluid maldistribution. • Examples covering different non-uniformity cases are taken to prove the accuracy of the plot. • Method is provided to simplify study of non-uniform flow into that with uniform flow. [ABSTRACT FROM AUTHOR]

In this paper a numerical investigation on shell-and-tube heat exchanger (STHE) with three configurations of novel segmental curved baffles (convex core baffles/convex peripheral baffles (CB1), convex peripheral baffles/convex core baffles (CB2), and convex peripheral baffles/concave core baffles (CB3)) for thermohydralic performance analysis and enhancement is presented. STHE with conventional straight baffles (SB) is also numerically studied for comparison. Each baffle configuration was tested under different Reynolds number in shell side (Re c) (17693–30331) based on the overall heat transfer coefficient (U), effectiveness (ε), and number of transfer unit (NTU). Also, pressure loss across the shell side is calculated to estimate the energy loss from the system due to the suggested design. In addition, the influence the inlet cold fluid temperature, baffle cutting ratio, and baffle spacing are tested, analyzed and discussed. The results showed that CB3 configuration results in a significant augmentation of the heat exchanger performance compared with the others two configurations for all investigated cases. The enhancements in U , ε, NTU by using CB3 configuration compared to SB are 48.98%−51.31% and 22.92%–23.62% and 48.98%–51.31% respectively, with decreasing in pressure loss by 12.40%–11.22%. [ABSTRACT FROM AUTHOR]

Finned tube heat exchangers are among the most common types of heat exchangers that are used in engineering applications. Most of the tubes used in finned-tube heat exchangers are of circular type, but the oval tubes have more desirable features as they impose lower pressure drops with approximately the same amount of heat transfer. In this paper, a finned oval tube is studied experimentally and numerically to obtain temperature fields around the tube at different locations. Moreover, several 3D numerical simulations are performed to study the effects of fin length, fin spacing and diameter ratio of the tube cross section. Based on the obtained results, fin spacing of 1 mm is the optimum at all tube aspect ratios and fin lengths when Reynolds number is low while at high Reynolds numbers, fin spacing of 3 or 4 mm is the better choice. As to the fin length considerations, for a short fin, a spacing of 3 or 4 mm is the best choice at high Reynolds numbers. For larger fin lengths, sensitivity to fin spacing is less but a fin spacing of 3 or 4 is preferable at high Reynolds numbers. • The excellence of oval tubes with rectangular fins to circular tubes with rectangular fins in certain cases is observed. • Fixing major tube diameter and increasing tube aspect ratio culminates in increased pressure loss and decreased heat transfer. • Smaller fin spacing is desirable at low Reynolds numbers for all cases. • Larger fin spacing is desirable for high Reynolds numbers when tube aspect ratio and fin length are smaller. [ABSTRACT FROM AUTHOR]