Offset Strip Fins Research Articles

Energy and exergy analysis of microchannel central solar receivers for pressurised fluids

Within the new generation of advanced central solar receivers, microchannel pressurised gas receivers are emerging as reliable and efficient alternatives to operate at high temperatures and pressures. This paper presents an optimisation and comparative analysis of different compact plate-fin type structures, constituting the receiver’s absorber panels, classified according to the type of fin arrangement inside: plain rectangular, plain triangular, wavy, offset strip, perforated, and louvred fin. A versatile thermo-fluid receiver model is implemented, allowing simple variation of characteristic geometric parameters of each structure. Exergy efficiency is chosen as the optimisation function, as it considers both heat and pressure losses.The framework of the analysis is set by the receiver’s boundary conditions, operating at the design point conditions of a solar thermal power plant. For each compact structure, the optimal configuration is determined, providing interesting findings that have not been reported in the state-of-the-art to date. Although all geometries show good thermal performance, the perforated and plain rectangular configurations demonstrate the best exergy efficiencies of 59.21% and 58.80%, respectively, favouring taller and narrower channels. This analysis methodology could be seamlessly extrapolated to other gases and working conditions, owing to the thermo-fluid model’s versatility, to reveal the optimal configuration for each case.

Experimental and numerical investigation on conjugate performance of fan and heat exchanger of helicopter oil cooling system

• The overall performance of axial fan was studied with two flowrate adjusting modes. • HEs with different fin structures and flow lengths were designed and fabricated. • Coupling performances of fan and different HEs were studied experimentally. • The flow field of oil cooling system at different air flow rate was simulated. • The performance curves obtained by two modes form a hysteresis region. In this article, a combined experimental and numerical study has been performed to investigate the operating performance of axial fan and oil cooling system. A test rig was established, and six types of plate-fin heat exchangers (HEs) with offset strip and rectangular fins and three different flow lengths of 30 mm, 60 mm and 90 mm at side were designed and manufactured. The performance of an axial fan with front guide vane was experimentally studied by two different adjusting modes: gradually increasing and decreasing air flow rate. The conjugate performances of the axial fan and different HEs were discussed in detail. Moreover, a three-dimensional (3D) model was developed to investigate the flow distribution of the system including fan and 30 mm offset strip fins HE at different flow rates. The results show that: (1) the total pressure performance curve of the axial fan under two adjusting modes could form a hysteresis region near the stall boundary. In the hysteresis region, the fan performance curves showed significant difference under both adjusting modes; (2) when the offset strip fins HE with large flow resistance is considered, the system could have two theoretical working points in the hysteresis region. For the case of HE with 90 mm offset strip fins, the flow rates of the system at two theoretical working points were 25.1 m 3 /min and 34.2 m 3 /min, and the heat transfer capacity of the HE were 23.1 kW and 27.5 kW, respectively. In the current experiment, it was found that the system operated at the point with smaller flow rate; (3) when the HE flow resistance exceeded a certain value, the boundary layer separation of the airflow could occur at the rotor blade. The separation had a small effect on the inlet airflow due to its turbulence kinetic energy was low and basically the same at each blade passage. Therefore, the system did not surge or stall at small flow rate.

Nusselt number for steady periodically developed heat transfer in micro- and mini-channels with arrays of offset strip fins subject to a uniform heat flux

In this work, the Nusselt number is examined for periodically developed heat transfer in micro- and mini-channels with arrays of offset strip fins, subject to a constant heat flux. The Nusselt number is defined on the basis of a heat transfer coefficient which represents the spatially constant macro-scale temperature difference between the fluid and solid during conjugate heat transfer. Its values are determined numerically on a single unit cell of the array for Reynolds numbers between 1 and 600 and fin height-to-length ratios below 1. Two combinations of the Prandtl number and the thermal conductivity ratio are selected, corresponding to air and water. It is shown that the Nusselt number correlations from the literature mainly apply to air in the transitional flow regime in larger conventional channels if the wall temperature remains uniform. As a result, they do not correctly capture the observed trends for the Nusselt number in micro- and mini-channels subject to a constant heat flux. Therefore, new Nusselt number correlations, obtained through a least-squares fitting of 2282 numerical simulations, are presented for air and water. The suitability of these correlations is assessed via the Bayesian approach for parameter estimation and model validation. The correlations respect the observed asymptotic trends and limits of the Nusselt number for all the geometrical parameters of the offset strip fins. In addition, they predict a linear dependence of the Nusselt number on the Reynolds number, in good agreement with the data from this work. Nevertheless, a detailed analysis reveals a more complex scaling of the Nusselt number with the Reynolds number, closely related to the underlying flow regimes, particularly the weak and strong inertia regimes. Finally, through 62 additional simulations, the influence of the material properties on the Nusselt number is illustrated and compared to the available literature.

전기자동차 배터리 냉각판 수냉 유로에 설치되는 오프셋 스트립 휜의 전열 및 수력 성능 해석

Thermal management of Lithium-ion batteries(LIB) is one of critical factors for ensuring the reliable operation of electric vehicles. For managing multiple LIB modules, a common cold plate is often mounted at the case of battery pack to achieve better thermal performance for the emission of higher thermal loads as well as more uniform temperature distribution. In this study, we proposed a new cold plate employing offset strip-fins (OSFs) cores, evaluating thermal and hydraulic performance with the objective functions of maximizing heat flux with modeling ideal fin theory. Shape optimization method is applied to obtain the optimal shape parameter of fin height and thickness for higher heat transfer rate with small increase of pressure drop. The performance of optimized design is compared with both the baseline no-fin design and the reference commercial OSFs cold plate. The maximum temperature difference is reduced 62.6% from the baseline model, so the uniformity of temperature is shown improved with the present new design as well as the remarkable reduction of pressure drop in the optimization.

Overall Pressure Loss and Heat Transfer Performance of Additively Manufactured Offset Strip Fins Used in Compact Heat Exchangers

Abstract Preliminary heat exchanger design relies heavily on correlations for overall heat transfer and pressure drop performance, particularly for heat transfer augmentation features such as fins. Extensive work has been performed by the research community to develop these correlations for the numerous designs. However, with the new technology of metal additive manufacturing and the resultant surface roughness, the traditional correlations and design considerations related to performance need to be adjusted. As a result, two metal additively manufactured offset strip fin heat exchanger geometries with different fin spacing are studied for heat transfer and pressure drop performance and compared with traditional correlations. Deviations between the additively manufactured geometries and previous correlations for smooth fins are found and are further amplified as the surface roughness-to-hydraulic diameter ratio is increased. Furthermore, the surface roughness from the additive process results in a constant friction factor behavior at high Reynolds numbers, which is unlike behavior observed for conventionally manufactured fins. Considerations for both the laminar and turbulent flow regimes are needed for correct performance prediction. A final offset strip fin geometry with a change in the fin spacing every third of the way through the flow path is tested. This study found that the orientation of fin spacing, wider spaced to tightly spaced or tightly spaced to wider spaced, did not have a significant effect on pressure drop or heat transfer. However, the study found a method for predicting the performance which will become important as additive manufacturing increases the complexity of heat exchanger designs.

Boiling of R290+CF3i mixture inside an offset strip fin plate heat exchanger

In this study, evaporation characteristics of mildly flammable R290 + CF3i mixture (35 % + 65 % by weight) were conducted experimentally in an offset strip fins (OSSF) embedded plate heat exchanger (PHEX). Various operating conditions, namely, saturation temperature (Tsat), heat flux (q), inlet vapor quality (xin), and mass flux (G), were considered in the range of 5–15 °C, 4–10 kW m−2, 0.1–0.8, and 40–80 kg m−2 s−1, respectively. From this study, it was found that nucleate boiling was quenched at lower xin regardless of the operating conditions, although the boiling process was led by convection at high xin. Except at higher G, there was a presence of dry-out at a mean x ranging between 0.36 and 0.50, while the two-phase frictional pressure drop increased steadily with mean x. The heat transfer coefficient was largely augmented with increments in G of 50.15 %, followed by increments in q and T of 15.1 % and 12.3 %, respectively. Similarly, the two-phase frictional pressure drop was increased by 12.3 % and 175.4 % with increases in q and G, respectively, although it diminished by 21.9 % with Tsat. Compared to R1234yf, the mean HTC of R290 + CF3i was 6 % lower with 7 % increased TP-FPD. Additionally, Nusselt number and friction factor correlations have been developed with excellent predictability of 7 % and 12 % of mean absolute error, respectively.

Heat transfer and pressure drop characteristics of R1234yf during evaporation in a plate heat exchanger with offset strip fins: An experimental study

Plate heat exchangers (PHEs) have been suggested for use in secondary loop air conditioning systems in automobiles operated with flammable refrigerants, such as R1234yf. In this study, the evaporation heat transfer coefficient (HTC) and two-phase frictional pressure drop (FPD) of R1234yf in a brazed PHE with offset strip fins were investigated. The mass flux in the experiment was 40–80 kg m−2 s−1, heat flux was 4–10 kW m−2, vapor quality at the inlet was 0.1–0.8, and saturation temperature was 5–15°C. Prior to the primary analysis, a coolant-to-coolant test was performed using a modified Wilson plot approach to estimate the multiplier (C) and the Reynolds number exponential (n). The observed C and n values were 0.868 and -0.496, respectively, with an R2 value of 0.977. The main results demonstrated an increase in the evaporation HTC with an increase in vapor quality in the initial stages, followed by a subsequent decrease, forming a peak at a mean vapor quality of 0.35–0.41. The nucleate and convective heat transfer regimes were dominant during the evaporation of R1234yf at lower and higher mean vapor qualities, respectively. Evidently, the evaporation HTC was significantly influenced by the mass flux and vapor quality compared with the heat flux and saturation temperature. Furthermore, the two-phase FPD continuously increased with the mass flux, vapor quality, and heat flux, although it decreased with the saturation temperature. As a result, new correlations are proposed herein for forecasting the Nusselt number and friction factor of R1234yf with average absolute errors of ±12% and ±15%, respectively.

A comprehensive design and optimization of an offset strip-fin compact heat exchanger for energy recovery systems

• Parametric, sensitivity, and optimization of compact heat exchanger is presented. • A normalized sensitivity analysis is used to identify the critical parameters. • Geometric parameters and fluid velocity appeared as the most influential parameters. • The optimization reduced the operational cost by 78%, and the total cost by 77%. Energy recovery in conventional thermal systems like power plants, refrigeration systems, and air conditioning systems has enhanced their thermodynamic and economic performance. In this regard, compact heat exchangers are the most employed for gas to gas energy recovery because of their better thermal performance. This paper presents an economic optimization of a crossflow plate-fin heat exchanger with offset strip fins. A detailed software-based numerical code for thermal, hydraulic, economic, and exergy analysis is developed for three fin geometries. Genetic Algorithm, parametric, and normalized sensitivity analyses are used to discover the most influential parameters to optimize the total cost. The parametric study showed that with the increase of mass flow rates and plate spacing, outlet stream cost and operating cost increased due to the rise in pressure drops. Finally, the optimization reduced the operational cost by ∼78.5%, stream cost by ∼64.5%, and total cost by ∼76.8%.

Experimental measurements and mathematical modeling of cold plate for aviation thermal management

This study, which has been motivated by the recent applications of the cold plate device in aviation thermal management, reports on physics-based mathematical models derived from the conservation laws of mass, momentum, and energy, and empiricism-based models. One of the objectives of the present work is to report on an elaborate and successful experimental work carried out on an additively manufactured device for the purpose of rigorously validating the numerical predictions. The excellent agreement between the numerical predictions and measured performance provides much needed confidence in the implementation, in the software package, of the offset-strip fin passage correlations, as well as in the software implementation of user-defined wavy fin correlations for aerospace heat exchangers and cold plates operating with ram air at Reynolds numbers below 8000. The contributions of this work can also be found in the development of a new and accurate thermal-hydraulic analysis procedure, referred to in this paper as plate-fin analogy. Results from this procedure are compared with those from thermal resistance network. The comparative study in this paper of the bulk and discrete enthalpy flux method is also new, as is the relative assessment of four off-set strip fin thermal-hydraulic models.

Experimental analysis of pressure drops and heat transfer coefficient during upward two-phase flow of R-134a/R-245fa mixture in an offset-strip finned passage

The current study focuses on an experimental investigation of upward vertical two-phase flow of binary mixtures in complex geometries composed of offset-strip fins. The flow boiling of a mixture containing 52% mol/mol of R-134a and 48% mol/mol of R-245fa was visualized. The ranges of mass flux and heat flux investigated were 11 – 25 kg.m−2.s−1 and 9 – 23 kW.m−2 respectively. The test section is an offset-strip finned passage whose hydraulic diameter is 1.75 mm. The operating pressure ranges from 2 to 5 bar absolute. The temperature glide of the used binary mixture ranges from 14 to 16 °C. Different flow regimes were identified along the test section such as bubbly, slug and annular regimes. The impact of strip-fins and mass flux on flow maps was analysed in this study. The pressure drop was analyzed and the obtained data were gathered to propose a new correlation for R-134a/R-245fa mixture in offset-strip fins. The local heat transfer coefficient was calculated and it has been found that nucleate boiling was the predominant mode of heat transfer.

Heat Transfer and Fluid Flow Characteristics in a Long Offset Strip Fin Channel by Using TiO2-Water Nanofluid

Heat Transfer and Fluid Flow Characteristics in a Long Offset Strip Fin Channel by Using TiO2-Water Nanofluid

Heat transfer factor j and friction factor f correlations for offset strip fin and wavy fin of compact plate-fin heat-exchangers

A unique model of heat transfer factor "j" and friction factor "f" are developed for different types of fins, which are rectangular offset strip fin, rectangular wavy fin compact heat exchangers by considering the geometric variables such as plate space (Hp), fin pitch (Pf), hydraulic diameter (Dh), wave amplitude (Aa), fin length (λ), and Reynolds number (Re). The proposed j and f models provided 98% of experimental data points within ±20% and 82.3% of experimental data points within ±10% accuracy for OSF fins. Also, they provided 100% of Kays and London's experiential data points within ±5% accuracy for wavy fins. These proposed j and f models for a wide range (120–10000) of Reynolds number flows, including laminar and turbulent flows. j model provides maximum RMSPE 15% and 2% accuracy values for PFHE offset strip fins and wavy fin geometries, respectively. f model provides maximum RMSPE 17% and 3% accuracy values for PFHE offset strip fins and wavy fin geometries, respectively. Based on RMSPE and experimental data analysis, proposed j and f models were validated and better than existing models for offset strip fins and wavy fins.

Friction factor for steady periodically developed flow in micro- and mini-channels with arrays of offset strip fins

In this work, the friction factor for steady periodically developed flow through micro- and mini-channels with periodic arrays of offset strip fins is analyzed. The friction factor is studied numerically on a unit cell of the array for Reynolds numbers ranging from 1 to 600, and fin height-to-length ratios below 1. It is shown that the friction factor correlations from the literature, which primarily focus on larger conventional offset strip fin geometries in the transitional flow regime, do not predict the correct trends for laminar flow in micro- and mini-channels. Therefore, a new friction factor correlation for micro- and mini-channels with offset strip fin arrays is constructed from an extensive set of numerical simulations through a least squares fitting procedure. The suitability of this new correlation is further supported by means of the Bayesian approach for parameter estimation and model validation. The correlation predicts an inversely linear relationship between the friction factor and the Reynolds number, in accordance with our observation that a strong inertia regime prevails over nearly the entire range of investigated Reynolds numbers. Yet, through a more detailed analysis, also the presence of a weak inertia regime and a transitional regime is identified, and the transitions from the strong inertia regime are quantified by means of two critical Reynolds numbers. Finally, the new correlation also incorporates the asymptotic trends that are observed for each geometrical parameter of the offset strip fin array, and whose origins are discussed from a physical perspective.

Numerical investigation of heat transfer and pressure drop characteristics in an offset strip fin heat exchanger

This paper presents a numerical simulation to determine the air-side heat transfer and the pressure drop characteristics of a flat tube heat exchanger with offset strip fin. The effects of the fin bending ratio such as 29%, 36%, 44%, 50%, and the fin spacing such as 2.10 mm, 2.35 mm, 2.60 mm on the performance of the heat exchanger are studied by using a commercial CFD software. The air having constant viscosity, thermal conductivity, and density enters the heat exchanger at 298 K and the wall temperature of the strip fins is considered as constant at 314 K. Variations of the heat transfer coefficient and the pressure drop in the airside are presented with respect to the frontal air velocity while Colburn j-factor and the friction factor f are presented with respect to the airside Reynolds number ranging from 200 to 1200. Finally, the thermal-hydraulic performance of all investigated cases is compared by using the volume goodness factor, j/f 1/3. The results show that the air-side heat transfer coefficient and the pressure drop increase when the frontal air velocity ascends. The air-side heat transfer coefficient decreases with the increase of fin spacing. The fin bending ratio does not have a significant effect on the pressure drop in the considered fin spacing. Both the Colburn j-factor and friction factor reduce with the increment of Reynolds number and fin spacing.

Experimental investigation of the thermal performance in a single-component two-phase flow in multistream multi-fluid plate-fin heat exchangers

This paper focuses on evaluating the effects of flow rate and temperature changes of hot and cold fluid flows in multistream multi-fluid plate-fin heat exchangers with a single-component two-phase flow in the middle heat exchanger. This three-fluid heat exchanger included three layers with offset strip and wavy fin configurations. In the adjacent heat exchanger with offset strip fins, hot and cold fluids were flowing, while the middle heat exchanger with wavy fins contained a single-component two-phase flow (condensed vapor). The Nusselt number, friction factor, the thermal performance factor, and the heat transfer surface contribution of the middle heat exchanger with each of the adjacent heat exchangers, were obtained using the measured experimental data. The results indicate that the two-phase Reynolds number was between 500 and 1800, and the vapor quality varied in the range of 0.17–0.56. Also, the heat transfer surface contribution of the adjacent cold and hot fluids was between 0.18 and 0.5. The two-phase flow in the wavy channel has a wavy and stratified pattern. The examinations showed that raising the Reynolds number of hot and cold fluids, respectively, led to maximum 13.8 % and 9.5 % of reduction in the vapor quality, 8.8 % and 9 % of increase in the two-phase Nusselt number, and 18 % and 12 % of reduction in the average value of friction factor. The domain change in the two-phase vapor quality, the two-phase fluid heat transfer surface contribution, and the flow pattern determination parameters including the dimensionless mass parameter and the Lockhart-Martinelli parameter was 33.67 %, 80.81 %, 32.72 %, and 19.09 %, respectively. As the Reynolds number of the cold fluid was changed, the variation domain of these parameters was higher compared to when the Reynolds number of the hot fluid was altered. Increasing the dimensionless temperature of the hot and cold raised the vapor quality by 12–16 %. The experimental results revealed that by decreasing the temperature of adjacent fluids and increasing their Reynolds number, the thermal performance factor of the two-phase flow was increased.

Numerical and experimental investigation on a latent heat thermal storage device featuring flat micro heat pipe arrays with offset strip fins

In this paper, a latent heat thermal storage device is designed and manufactured. The device uses flat micro heat pipe arrays as the core heat transfer element, multichannel flat pipes as the heat transfer fluid channel to provide and remove heat, and offset strip fins to strengthen the heat exchange of the phase change material side. The performance of the device under different operating conditions was explored, and a 3D computational fluid dynamic model of the device is established. The model is simplified by using the method of equivalent thermal conductivity and equivalent specific heat capacity. The performance of a latent heat storage unit under small temperature difference is also studied. Results show that the inlet temperature of fluid has a great influence on the performance of the device. During charging process, when the fluid temperature changes from 63 °C to 73 °C, the charging power increases from 921.1 W to 1302.1 W (41.3%). During discharging process, when the fluid temperature decreases from 18 °C to 8 °C, the discharging power increases from 831.1 W to 1210.5 W (48.9%). Regarding the flow rate, when the fluid flow rate increases from 2 L/min to 3 L/min (50%), the average charging power increases from 1,069.2 W to 1,199.3 W (12.2%), and the average discharging power increases from 895.1 W to 1,068.3 W (19.3%). Simulation results show that the phase change material in the thermal storage unit can melt completely when the temperature difference is 4 °C, but the melting time is longer when the heat transfer temperature difference is less than 6 °C. Heat transfer temperature difference refers to the variance between the heat transfer fluid inlet temperature and the phase change material phase transition temperature. The main thermal resistance is concentrated in the part where multichannel flat pipes transfer heat to the flat micro heat pipe arrays and the phase change material side.

A novel condenser with offset strip fins on a mini channel flat tube for reducing the energy consumption of a household refrigerator

The aim of the study is to reduce the amount of refrigerant and energy consumption of a household refrigerator by conserving the desired temperature inside the fresh food and freezer compartments. In this study, two novel condensers with different offset strip fin configurations on a mini-channel flat tube are used instead of a conventional wire-on-tube condenser. The impact of this novel application on the performance of the refrigerator is investigated experimentally by following the norms defined by IEC 62552:2015. The tests are conducted in a wide range of refrigerant charges (48–64 g) with three different capillary lengths (2800, 3300, and 3800 mm). Alternative couples of refrigerant charge and capillary length for the novel condensers are determined to decrease the daily energy consumption under the level of the original system with the wire-on-tube condenser. One of the aims of the study is achieved by 5.7% of energy-saving when one of the novel condensers is coupled with a 3300 mm length capillary tube and 50 g of refrigerant. The other aim of the study related with the environmental effect is achieved by reducing 10.7% of the refrigerant amount with the application of the novel condenser with offset strip fins on a mini channel flat tube.

Physical Dimensions as a Design Objective in Heat Transfer Equipment: The Case of Plate and Fin Heat Exchangers

To incorporate exchanger dimensions as a design objective in plate and fin heat exchangers, a variable that must be taken into consideration is the geometry of the finned surfaces to be used. In this work, a methodology to find the surface geometry that will produce the required heat transfer coefficient and pressure drop to achieve the design targets was developed. The geometry of secondary surfaces can be specified by the fin density, which represents the number of fins per unit length. All other geometrical features, as well as the thermo-hydraulic performance, can be derived from this parameter. This work showed the way finned surfaces are engineered employing generalised thermo-hydraulic correlations as a part of a design methodology. It also showed that there was a volume space referred to as volume design region (VDR) where heat duty, pressure drop, and dimensions could simultaneously be met. Such a volume design region was problem- and surface-specific; therefore, its limits were determined by the heat duty, the pressure drop, and the type of finned surface chosen in the design. The application of this methodology to a case study showed that a shell and tube heat exchanger of 227.4 m2, with the appropriate fin density using offset strip-fins, could be replaced by a plate and fin exchanger with any combination of height, width, and length in the ranges of 0–0.58 m, 0–0.58 m, and 0–3.59 m. The approach presented in this work indicated that heat exchanger dimensions could be fixed as a design objective, and they could effectively be achieved through surface design.

Experimental and Computational Analysis of Additive Manufactured Augmented Backside Liner Cooling Surfaces for Use in Micro-Gas Turbines

Abstract Augmented backside cooling refers to the enhancement of the backside convection of a combustor liner using extended heat transfer surfaces to fully utilize the cooling air by maximizing the heat transfer to pumping ratio characteristic. Although film cooling has and still is widely used in the gas turbine industry, augmented backside cooling has been in development for decades now. The reason for this is to reduce the amount of air used for liner cooling and to also reduce the emissions caused by using film cooling in the primary zones. In the case of micro-gas turbines, emissions are of even greater importance, since the regulations for such engines will most likely become stricter in the following years due to a global effort to reduce emissions. Furthermore, the liners investigated in this paper are for a 10 kWe micro-turbine, destine for various potential markets, such as combined heat and power for houses, electric vehicle hybrids and even small unmanned aerial vehicles. The majority of these markets require long service intervals, which in turn requires the combustor liners to be under the least amount of thermal stress possible. The desire to also increase combustor inlet temperatures with the use of recuperated exhaust gases, which in turn increase the overall system efficiency, limits the cooling effectiveness of the inlet air. Due to all these reasons, an advanced form of augmented backside cooling would be of substantial significance in such a system. Currently, some very simple designs are used in the form of straight plain fins, transverse strips, or other similar geometries, but the creation of high heat transfer efficiency surfaces in such small sizes becomes very difficult with traditional subtractive manufacturing methods. When using additive manufacturing though these types of surfaces are not an issue. This paper covers the comparison of experimental results with conjugate heat transfer computational fluid dynamics (CFD) models and empirical heat balance models for two different additively manufactured (AM) liner cooling geometries and an AM blank liner. The two cooling fin geometries include a rotating plain fin and an offset strip fin. The liners were tested in an AM-built reverse flow radial swirl stabilized combustion chamber at a variety of operating conditions. During the experiments, the surfaces were compared using a thermal camera to record the outer liner temperature, which was viewed through a quartz outer casing. The experimental results showed that the cooling surfaces were effective at reducing the liner temperatures with minimal pressure losses for multiple operating points. Those results were then compared against the conjugate heat transfer CFD models and the empirical calculations used to design the surfaces initially. From this comparison, it was noticed both the CFD and empirical calculations under-predicted the wall temperatures. This is thought to be due to inaccuracies in the predicted flame temperatures and the assumed emissivity values used to calibrate the thermal imaging camera. Further uncertainties arise from the assumption of a constant air and hot gas temperature and mass flow along the cooling surfaces and the lack of data for the surface roughness of the parts.

Improved heat transfer and friction correlations of aluminum offset-strip fin heat exchangers for helium cryogenic applications

• Improved performance correlations of cryogenic helium through OSFs were proposed. • Comparative thermal–hydraulic performance at different temperatures were studied. • Accuracy and robustness of the correlations’ prediction ability were validated. • Sensitivity of Re , Pr and η f on j values in laminar and turbulent regions was analyzed. • Correlations were qualified for OSF performance prediction in helium refrigerators. Aluminum plate-fin heat exchangers (PFHEs) with offset-strip fins (OSFs) are widely used in modern helium liquefaction/refrigeration systems characterized by enhanced heat transfer abilities and high compactness. This study numerically and experimentally investigated the thermal–hydraulic characteristics of cryogenic helium gas flowing through various OSF channels. Through the comparative tests conducted by well-validated numerical model as well as the contrast experiments respectively at room and nitrogen liquefaction temperatures, the results indicated that the OSF performance was not only affected by fin geometrical structures, but also deeply influenced by additional effect factors under actual cryogenic operating conditions. Based on the orthogonal table with 0.077 ⩽ α ⩽ 1.103 , 0.008 ⩽ δ ⩽ 0.15 and 0.029 ⩽ γ ⩽ 0.333 , the heat transfer and pressure drop performance of OSFs was systematically studied at a wide range of Re number ( 200 ⩽ Re ⩽ 12000 ) in three common refrigeration stages (300 K @ 12 bar, 77 K @ 6 bar and 20 K @ 2 bar). To improve the prediction accuracy of the correlations at low temperatures, the non-dimensional Pr Number and iteratively-solved fin efficiency η f that considering the thermophysical property variations of fluid and metal were introduced in the correlations of Colburn j and Fanning friction f factors. Compared with the experimental data, the proposed model had strong robustness of prediction ability, with the predicted errors of 11.33% and 22.49% respectively for j and f factors. Additionally, the effect of Pr number and η f occupied different weights in the j factors from laminar to turbulent flow. The sensitivity of the heat transfer performance with respect to each condition parameter was also discussed. Different from the most of the existing correlations derived from the fitted data based on common steam-to-air tests, the proposed correlations were more qualified for predicting the thermal–hydraulic performance of OSF channels in PFHEs through the cool-down process in helium refrigerators.