Internal Heat Transfer Enhancement Research Articles

Flow and Heat Transfer Experimental Study for 3D-Printed Solar Receiving Tubes With Helical Fins at Internal Surface

Abstract 3D-printing technology was applied to fabricate novel solar thermal collection tubes that have internal heat transfer enhancement fins and external surfaces with high solar absorptivity and low emissivity due to the ability to use different materials in one tube. Helical fins were selected to introduce circumferential flow and thus minimize the circumferential temperature difference of the tube that receives sunlight on one side. The structures of the helical fins were previously optimized from computational fluid dynamics (CFD) analysis with the objective of low entropy production rate by looking for high heat transfer coefficient and relatively lower pressure loss. High-temperature alloy, Inconel-718, was used to 3D print the tubes, which can resist corrosion for the potential application of molten chloride salts as heat transfer fluid. Experimental tests were carried out using water as the heat transfer fluid with the high heat flux provided by a tubular furnace heater. The tested Reynolds number ranges from 3.9 × 103 to 6.1 × 104. Heat transfer coefficients of up to 2.8 times that of the smooth tube could be obtained with the expense of increased pressure loss compared to that of the smooth tube. The total system entropy generation can be significantly reduced due to the benefit of heat transfer enhancement that is greater than the expenses of the increased pressure loss. The experimental results of the 3D-printed heat transfer tubes confirmed the CFD-based results of fin optimization. The novel heat transfer tube is recommended for application in concentrating solar power systems.

A comparative study of internal heat transfer enhancement of impingement/effusion cooling roughened by solid rib and slit rib

In the present study, we conducted a conjugate heat transfer (CHT) analysis for double-wall cooling with impingement and effusion, incorporating various types of ribs, using the Reynolds-averaged Navier–Stokes (RANS) method and the modified shear stress transport (SST) turbulence closure model (SST-KIC), accounting for the Kato-Launder modification (K), intermittency (I), and crossflow (C) transition effects. We comprehensively discussed the impact of slit type (parallel, inclined, convergent, and divergent), open-area ratio (β = 5%, 20%, and 40%), and jet Reynolds number on the turbulent flow and heat transfer in a double-wall cooling with slit ribs. Our findings indicated that the introduction of slit ribs significantly improved heat transfer and its uniformity on the target wall, albeit with a slight increase in pressure loss. The overall Nusselt number and thermal-hydraulic performance (THP) in cases with slit ribs gradually decreased with β, yet remained up to 17% and 13% higher than those observed on a smooth target wall. Notably, the open-area ratio of the slit rib exhibited a more pronounced effect on heat transfer over the target plate. For the divergent slit rib within the Reynolds number range of 4000–16 000, the heat transfer enhancement ratio reached the highest value at β of 0.05. In addition, we computed the entropy production caused by fluid friction and heat transfer, as well as the overall entropy production in double-wall cooling at different β and Re. The analysis revealed that the slit rib target plate performed better than the solid rib target plate, showing a distinct advantage in terms of total entropy production.

Summer thermal and energy performances assessment of a modular hydronic thermal barrier wall for ultra-low energy buildings - A field experimental study

Hydronic thermal barriers with ultra-low carbon emission attributes are progressively gaining prominence in the competition of thermal insulation solutions for opaque envelopes of low-energy buildings due to their excellent thermal performance and low-grade energy utilization potential. This paper introduces a novel approach involving a modular hydronic thermal barrier wall (MHTB) that incorporates preset pipe cavities and replaceable filling material. This proposition seeks to address the challenges confronted by conventional hydronic thermal barrier walls (CHTBs), specifically in terms of internal heat transfer enhancement and rapid assembly/dismantling. On-site thermal performance tests and energy-saving evaluations were conducted in summer conditions based on three scaled-down rooms with MHTB, CHTB and a conventional external insulation wall as the south walls. The results showed that the MHTB could significantly reduce heat passing through the south wall in summer, resulting in a decreased cooling load attributed to the wall and subsequently lowering indoor temperatures. At a charging temperature of 17.0 °C, the MHTB exhibited a reduction of 11.3 °C in interior surface temperature compared to the conventional external insulation wall, accompanied by a corresponding increase of 2.76 kWh·m−2 in cumulative heat flux. In addition, the MHTB could effectively mitigate inhomogeneous heat transfer within the wall, enhancing the overall heat transfer effect. Across the four investigated conditions, the MHTB with a steel grit-sand mixture as the filling material demonstrated an approximate 4.7% increase in cumulative heat flux and a 12.2% enhancement in cumulative cooling quantity compared to the CHTB. Overall, the findings substantiated the efficacy of the MHTB system and offered a crucial point of reference for advancing research in optimizing the performance and modular construction of hydronic thermal barrier walls.

Numerical decoupling of the effect of internal cooling and external film cooling on overall cooling effectiveness

In modern aero-engine turbine blades, the coolant flows from the internal cooling channel and out through the film hole, forming a coolant coverage over the blade surface to isolate the high-temperature gas. Essentially, the cooling system can be divided into parts with internal cooling and external film cooling; in particular, external film cooling includes coolant coverage (film cooling) and bore cooling (cooling within the film hole). Accordingly, it is important to quantitatively analyze the effects of these three cooling parts (internal, bore, and film cooling) on the overall cooling effectiveness, which can help improve the design of turbine blades. To this end, in this study, the numerical decoupling method was used to investigate the relationship between the overall cooling effectiveness of two plate cooling structures and three cooling parts. Considering blowing ratios of 0.25, 0.5, 1, and 1.5, it was found that the blowing ratio and film hole position have considerable impacts on the overall cooling effectiveness of a smooth model; however, the overall cooling effectiveness of an impingement-effusion model was deemed insensitive to the film hole position. Furthermore, in the case of a small blowing ratio (M = 0.25), the internal heat transfer enhancement caused by impinging jets was less than that due to increased coolant flow rates. In the regions located upstream of each film hole exit, bore cooling had significant effects on the overall cooling effectiveness; particularly, these effects were more pronounced in the smooth model. Moreover, for a blowing ratio of 0.25, film cooling was dominant across all the cooling parts. With an increase in the blowing ratio, the influence of bore cooling grew rapidly, gradually replacing film cooling. Additionally, as the blowing ratio increased to 1.5, the effect of internal cooling exceeded that of film cooling. This work and its results are expected to serve as guidance for the design of turbine blade cooling structures.

Experimental investigation on annular metal hydride reactor for medium to large-scale hydrogen storage applications

In the present study, an annular metal hydride reactor equipped with internal radial fins was designed, fabricated and experimented for absorption and desorption characteristics under different operating conditions. The reactor was filled with 9 kg LaNi5 powder. The alloy absorbed 105.57 g of hydrogen in 1184 s under a supply pressure of 10 bar while the inlet temperature and flow rate of the heat transfer fluid were maintained at 25°C and 2.5 lpm, respectively. At the same time, for a fixed absorption mass of 111 g of hydrogen, the alloy desorbed 110 g of hydrogen in 1590 s at an inlet temperature of 60°C. The developed metal hydride reactor offered a system gravimetric and volumetric storage densities of 0.73% and 20.4 kg of H2 per m3, respectively. Further, the performance of the new reactor was compared to published results for an embedded cooling tube reactor with an outer cooling jacket. The comparison results suggested that the internal heat transfer enhancement of the metal hydride bed resulted in a significant improvement in the absorption (about 56%) and desorption (about 44%) performance than the reactor with multiple cooling tubes. The results also revealed better absorption and desorption rates with the water flow rate of 2.5 lpm (annular metal hydride reactor) instead of a very high flow rate of 20 lpm (embedded cooling tube reactor with an outer cooling jacket), saving a significant amount of pumping power. Finally, a modular design based on the annular metal hydride reactor is proposed for medium to large-scale hydrogen storage applications.

A numerical investigation of dimple effects on internal heat transfer enhancement of a double wall cooling structure with jet impingement

Purpose The dimple is adopted into a double wall cooling structure which is widely used in hot gas components to increase the heat transfer effects with relatively low pressure drop penalty. The purpose of this paper is to study the effect of dimple depth and dimple diameter on the target surface heat transfer and the inlet to outlet friction factor. Design/methodology/approach The study is carried out by using the numerical simulations. The impingement flow is directly impinging on the dimple and released from the film holes after passing the double wall chamber. The ratio between dimple depth and dimple diameter is varied from 0 to 0.4 and the ratio between dimple diameter and impingement hole diameter is ranging from 0.5 to 3. The Reynolds number is between 10,000 and 70,000. Results of the target surface Nusselt number, friction factor and flow structures are included. For convenience of comparison, the double wall cooling structure without the dimple is considered as the baseline. Findings It is found that the dimple can effectively enhance the target surface heat transfer due to thinning of the flow boundary layer and flow reattachment as well as flow recirculation outside the dimple near the dimple rim especially for the large Re number condition. However, the stagnation point heat transfer is reduced. It is also found that for a large dimple depth or large dimple diameter, a salient heat transfer reduction occurs for the toroidal vortex. The thermal performance indicates that the intensity of the heat transfer enhancement depends upon the dimple depth and dimple diameter Originality/value This is the first time to adopt a dimple into a double wall cooling structure. It suggests that the target surface heat transfer in a double wall cooling structure can be increased by the use of the dimple. However, the heat transfer characteristic is sensitive for the different dimple diameter and dimple depth which may result in a different flow behavior

Experimental study of an externally finned tube with internal heat transfer enhancement for phase change thermal energy storage

After having presented the design of a latent heat thermal energy storage system (LHTESS) for district heating, experimental results of a vertical tube-in-shell LHTESS are discussed. The tube is radially finned on its external wall to enhance the heat transfer in the phase change material. The test rig is operated with flow conditions corresponding to the proposed design. As the internal flow of heat transfer fluid (HTF) appears to be laminar and is highly influenced by buoyancy forces, which results in mixed convection regime, cross-sectional area reducers are installed inside the HTF tube in order to reduce the Rayleigh number and thus natural convection. Experimental results are presented for two finned tubes, with and without internal heat transfer enhancement respectively.

Numerical comparison of heat-fin- and metal-foam-based hydrogen storage beds during hydrogen charging process

We numerically studied the effects of additional components for internal heat transfer enhancement—that is, heat fins and metal foams—on the hydrogen charging performance. First, we applied a three-dimensional transient hydrogen absorption model developed in our previous works to two different experimental MHBs: one with heat fins and the other with metal foam. The simulation results were then compared with the experimental data measured during the hydrogen charging processes. The model predictions were found to be in good agreement with the experimental data in terms of the bed temperature evolution and hydrogen charging time. In addition, a parametric study was conducted on various MHBs with heat fins or metal foams that were designed to contain the same amount of ZrCo powder for the sake of comparison. The detailed analysis of the hydrogen absorption behavior with different MHB designs clearly demonstrated that superior MHB design should exhibit a higher effective conductivity in the metal powder region and lower overall thermal mass; these two advanced features can be achieved by using a metal foam with optimum porosity.

Heat Transfer Enhancement in a Rectangular (AR = 3:1) Channel With V-Shaped Dimples

An alternative to ribs for internal heat transfer enhancement of gas turbine airfoils is dimpled depressions. Relative to ribs, dimples incur a reduced pressure drop, which can increase the overall thermal performance of the channel. This experimental investigation measures detailed Nusselt number ratio distributions obtained from an array of V-shaped dimples (δ/D = 0.30). Although the V-shaped dimple array is derived from a traditional hemispherical dimple array, the V-shaped dimples are arranged in an in-line pattern. The resulting spacing of the V-shaped dimples is 3.2D in both the streamwise and spanwise directions. A single wide wall of a rectangular channel (AR = 3:1) is lined with V-shaped dimples. The channel Reynolds number ranges from 10,000–40,000. Detailed Nusselt number ratios are obtained using both a transient liquid crystal technique and a newly developed transient temperature sensitive paint (TSP) technique. Therefore, the TSP technique is not only validated against a baseline geometry (smooth channel), but it is also validated against a more established technique. Measurements indicate that the proposed V-shaped dimple design is a promising alternative to traditional ribs or hemispherical dimples. At lower Reynolds numbers, the V-shaped dimples display heat transfer and friction behavior similar to traditional dimples. However, as the Reynolds number increases to 30,000 and 40,000, secondary flows developed in the V-shaped concavities further enhance the heat transfer from the dimpled surface (similar to angled and V-shaped rib induced secondary flows). This additional enhancement is obtained with only a marginal increase in the pressure drop. Therefore, as the Reynolds number within the channel increases, the thermal performance also increases. While this trend has been confirmed with both the transient TSP and liquid crystal techniques, TSP is shown to have limited capabilities when acquiring highly resolved detailed heat transfer coefficient distributions.

Analytical approximation for solidification processes in PCM storage with internal fins: imposed heat flux

Uses of thermal energy storage systems have expanded notably in recent decades. In thermal energy systems, internal heat transfer enhancement techniques such as fins are often used because of the low thermal conductivity of the phase change materials (PCMs). In this paper, solidification of a PCM is studied in a rectangular storage with horizontal internal plate fins and an imposed constant heat flux on the vertical walls. A simplified analytical solution is presented and its results are compared to those for a numerical approach based on an enthalpy method. The fraction of solidified PCM in storage is calculated with the derived analytical model which determines how much of the storage is solidified after a certain time. The results show that the analytical model satisfactorily estimates the solid–liquid interface and the temperature distribution for the fin, which are useful in the design of PCM-based thermal energy storage or cooling systems.

Computational Analysis of Pin-Fin Arrays Effects on Internal Heat Transfer Enhancement of a Blade Tip Wall

Cooling methods are strongly needed for the turbine blade tips to ensure a long durability and safe operation. Improving the internal convective cooling is therefore required to increase the blade tip life. A common way to cool the tip is to use serpentine passages with 180-deg turns under the blade tip cap. In this paper, enhanced heat transfer of a blade tip cap has been investigated numerically. The computational models consist of a two-pass channel with a 180-deg turn and various arrays of pin fins mounted on the tip cap, and a smooth two-pass channel. The inlet Reynolds number is ranging from 100,000 to 600,000. The computations are 3D, steady, incompressible, and nonrotating. Details of the 3D fluid flow and heat transfer over the tip walls are presented. The effects of pin-fin height, diameter, and pitches on the heat transfer enhancement on the blade tip walls are observed. The overall performances of ten models are compared and evaluated. It is found that due to the combination of turning, impingement, and pin-fin crossflow, the heat transfer coefficient of the pin-finned tip is a factor of 2.67 higher than that of a smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 30%. Results show that the intensity of heat transfer enhancement depends upon pin-fin configuration and arrangement. It is suggested that pin fins could be used to enhance the blade tip heat transfer and cooling.

Approximate analytical model for solidification in a finite PCM storage with internal fins

In latent heat storage internal heat transfer enhancement techniques such as fins have to be used because of the low heat conductivity of the phase change material. Internal heat transfer enhancement is essential, especially in a solidification process where the main heat transfer mode is conduction. The aim of this paper is to present a simplified analytical model which predicts the solid–liquid interface location and temperature distribution of the fin in the solidification process with a constant end-wall temperature in the finned two-dimensional PCM storage. The analytical results are compared to numerical results and they show that the analytical model is more suited to the prediction of the solid–liquid interface location than the temperature distribution of the fin in the PCM storage. A new factor called the fraction of solidified PCM is also introduced. The fraction of solidified PCM easily gives a good picture of how much of the storage is solidified after a given time.

Analytical model for melting in a semi-infinite PCM storage with an internal fin

The most PCMs with high energy storage density have an unacceptably low heat conductivity and hence internal heat transfer enhancement techniques such as fins or other metal structures are required in latent heat thermal storage (LHTS) applications. Previous work has concentrated on numerical and experimental examination in determining the influence of the fins in melting phase change material. This paper presents a simplified analytical model based on a quasi-linear, transient, thin-fin equation which predicts the solid–liquid interface location and temperature distribution of the fin in the melting process with a constant imposed end-wall temperature. The analytical results are compared to the numerical results and they show good agreement. Due to the assumptions made in the model, the speed of the solid–liquid interface during the melting process is slightly too slow.

Effect of baffle size, perforation, and orientation on internal heat transfer enhancement

An experimental investigation of frictional loss and heat transfer behavior of turbulent flow in a rectangular channel with isoflux heating from the upper surface is presented for different sizes, positions, and orientations of inclined baffles attached to the heated surface. Both solid and perforated baffles are used. Inclined perforated baffle combines three major heat transfer techniques, e.g. boundary layer separation, internal flow swirls, and jet impingement. Results indicate that there exists an optimum perforation density to maximize heat transfer coefficients and this optimum perforation exhibits a strong jet impingement technique from the lower confined channel along with other enhancement techniques of heat transfer.