Chi HOU,Guangmeng YANG,Xiaopeng WAN,Jie CHEN

School of Aeronautics,Northwestern Polytechnical University,Xi'an 710072,China

KEYWORDS Forced convection;Heat transfer;Kagome;Pressure drop;Topology direction

Abstract The aim of this paper is to provide a comprehensive comparison of the thermal insulation and heat transfer performance for Kagome truss-cored lattice along two perpendicular orientations OA and OB.Three test conditions are conducted under forced air convection for the titanium sandwich panel fabricated by 3D printing technology.The thermo-fluidic characteristics are further explored by numerical simulation to reveal the underlying mechanisms of heat transfer enhancement.The results indicate that the orientation OB exhibits better thermal insulation than orientation OA under the identical temperature loading,while the latter outperforms the former by up to 20%higher overall heat transfer performance.In particular,the endwalls and lattice core in orientation OA achieve 9.7%and 22.5%higher area-averaged Nusselt number respectively than that in orientation OB for a given Reynolds number.The heat transfer superiority of orientation OA comes from the unique topology which induces the large scale spiral primary flows,facilitating the heat exchange between the cooling air and the surfaces of sandwich panel.However,the complex flow mixing leads to a maximum of 20%higher friction factor in orientation OA than that in orientation OB.

1.Introduction

In thermal management systems,the requirement for structures that satisfies the mechanical and thermal loadingbearing capabilities concurrently has become challenging due to the serious service environment.1-4For the situation,the lightweight Periodic Cellular Materials(PCMs)are considered as the most promising multifunctional media since they exhibit higher specific strength and stiffness in comparison with the stochastic cellular materials.4-7Additional, the distinct topologies of PCMs with high specific surface area provide the excellent heat dissipation capability.8,9Therefore,considerable efforts have been devoted to developing efficient morphology of PCMs to enhance the heat transfer performance,which mainly include woven textile,9-11pyramidal lattice,12-14tetrahedral lattice15-17and Kagome.18,19Amongst them, the Kagome lattice exhibits structural superiority over tetrahedral PCM with better isotropy under shear loading and better resistance to plastic bulking.20For a given porosity,the overall Nusselt number of Kagome PCM is about 38%higher than that of tetrahedral PCM.21In addition,the relatively simple geometrical topology of truss-cored Kagome is facilitated to be fabricated by using various manufacturing methods, e.g.,investment casting,22metal sheet folding,23metal wire weaving19and 3D printing,24etc.In view of these advantages,the Kagome lattice exhibits the promising potential for lightweight multifunctional and cost-effective applications.

Considering the inherent geometric anisotropy of Kagome lattice core,the heat transfer characteristics,especially under the influence of flow orientation have been massive reported by many researchers using both experimental and numerical simulations.Joo et al.25,26conducted the convective experiment for wire-woven bulk Kagome(WBK)and the results showed that both the heat transfer and pressure drop are obviously related to the airflow direction due to the geometric anisotropy of WBK.Moreover,Yang et al.27,28experimentally studied the effect of orientation on thermal insulation for WBK under natural convection;the results showed that the anisotropic topology also effects the thermal protection performance.Besides,a single-layer truss-cored Kagome lattice has been studied numerically by Yan compared with an X-type lattice with square cross-sectioned ligaments.It can be concluded that the Kagome lattice presents comparable heat transfer performance and less pressure drop to the X-type lattice.Krishnan et al.29conducted the numerical comparison of thermal and fluidic characteristics for tetrahedral,Kagome and pyramidal lattice;the effect thermal conductivity of PCM cores can be twice as good as that of stochastic cellular materials.Furthermore, the two types of sandwich panels between WBK and truss-cored Kagome lattice were compared systematically on heat transfer by Shen et al.30.They demonstrated that the truss-cored Kagome exhibits a 26%-31%higher overall Nusselt number than WBK with similar pressure drop.Since the truss-cored Kagome has high strength under compression,shear and bending load,which is similar to that of WBK,1,Hoffman18experimentally investigated the effect of flow orientation and core porosity on heat transfer characteristics for truss-cored Kagome.

As established by the aforementioned researches,the flow orientation has an evident effect on the thermal performance for geometric-anisotropy Kagome lattice.However,a comprehensive exploration of the heat transfer and thermal insulation performance corresponding to different flow directions for Kagome truss-cored lattice is still necessary to better understand this phenomenon.Furthermore,in order to obtain the underlying mechanisms of orientation effect on heat transfer enhancement,the detailed thermo-fluidic behaviors need to be classified as it dominates the heat exchange within lattice core.To this end,the present work presents the forced convective experiment for titanium Kagome truss-cored lattice sandwich panel fabricated by 3D printing technology.31Three test conditions including the variation of heating temperature,airflow velocity and thermal boundary condition are conducted sequentially to draw an adequate comparison of the thermal performance for different airflow orientations.Subsequently,the numerical simulation is validated and utilized to explore the fluid flow characteristics and their effects on local and overall heat transfer to reveal the mechanisms of different thermal performance caused by different airflow orientations.

2.Experimental investigation

2.1.Experimental specimen and apparatus

Fig.1 Kagome truss-cored lattice sandwich panel.

The Kagome truss-cored lattice sandwich panel made of titanium alloy TC4 is shown in Fig.1.A unit cell of the Kagome lattice is illustrated in Fig.1(b),with three trusses L1,L2and L3being intersected at the center vertex.The geometrical parameters depicted in Fig.1 are listed in Table 1,from which the relative density of Kagome lattice core is calculated at 0.02,corresponding to a core porosity of 0.98.Two perpendicular orientations OA and OB are selected for forced convective experiment and the corresponding frontal views are shown in Fig. 1(c) and (d), respectively. To minimize the sidewall effects,15the sandwich panel is fabricated with a relatively large in-plane size;the Kagome core contains ten unit cells along both the orientations OA and OB.The 3D printing technology is adopted to fabricate the lattice core and panels integrally,which can reduce the thermal contact resistance compared with conventional manufacturing methods;meanwhile, lead to a significant increase in heat transfer performance.9

Fig.2(a)presents a schematic illustration of the test facilities under the forced convective experiment.Compressed air from air tank is supplied into the entry section of the air hose.To ensure that well-conditioned laminar flow enters the sample,a transition channel is installed at the exit of the entry section,followed by a thin wire screen layer.Subsequently,two flow channels with length of 790 mm are placed upstream and downstream of the test section respectively to ensure the flow entering and leaving the test section steadily.

A quartz lamp composed of etched Inconel heating pipes sandwiched between two Kapton films is configured above the bottom substrate(named as heating surface),as shown in Fig.2(b).The quartz lamp is connected to a DC power supply and the applied thermal flux is controlled by an automatic system so that the heating temperature can keep to the specified value according to the test conditions.A photo of test section is presented in Fig.2(c)where the heating surface of specimen is painted black to absorb the radiate energy from quartz lamp better.Finally,in order to ensure the requirement of thermal insulation,the flow channel and side-walls are sealed by thermal baffles and subsequently insulated by asbestos cloth.

2.2.Test condition

Three test conditions are conducted in forced air convection along airflow orientation OA and orientation OB for Kagome lattice sandwich panel, as illustrated in Fig. 3. They are labelled Case A,Case B and Case C,respectively.For the Case A,various heat flux is applied on the heating surface to ensure the heating temperature ranges from 100°C to 500°C gradually with a temperature gradient of 100°C.Case B adopts the same method as Case A to impose the temperature loading,while converts the boundary condition of the upper facesheet(named the cooling surface)from heat insulation to nature convection,which is achieved by removing the thermal baffles of cooling surface.In above two test conditions,the thermal insulation performance under forced air convection is explored with the constant airflow velocity designed of 10.0 m/s.Finally,the heat transfer characteristic for the two airflow directions is studied in Case C where the inlet velocity of cooling air is changed from 5.0 m/s to 15.2 m/s,with the constant heating temperature of 300°C on heating surface and adiabatic boundary on cooling surface.

The volume flow rates and temperatures are measured in experiment.A flow meter is installed at the entry section to obtain the inlet flow rate.Ten K-type thermocouples with the precision of±1.5°C are divided into two groups(N1-N5 and N6-N10)then attached on the cooling surface andheating surface,respectively.Fig.3 presents the distribution of the five thermocouples on the cooling surface.The facesheet is partitioned into four subdomains uniformly;four thermocouples are located at the center of each subdomain while the thermocouple N5 is positioned at the center of the cooling surface.The test status is considered to reach stabilization once the temperature monitored by the thermocouple N10 at the center of heating surface ranges within 0.2°C in 3 min.

Table 1 Geometrical parameters of the Kagome truss-cored lattice sandwich panel.

Fig.2 Schematic and photos of test setup.

2.3.Experimental results

The thermal insulation performance for orientation OA and OB can be evaluated in term of the upper temperature distribution under the identical temperature loading.The corresponding measured results of Case A and Case B are plotted in Fig.4 where the upper temperature and heating temperature are determined by the average value of thermocouples N1-N5 and N6-N10,respectively.It can be seen that in the entire range of heating temperature considered in the present experiment,the cases along orientation OB exhibits the lower mean temperature than that along orientation OA in both adiabatic and convective boundary condition.Furthermore,the temperature difference becomes obvious with the increase of heating temperature,which indicates that the Kagome lattice sandwich panel has the better thermal insulation when air flows along orientation OB,especially at high temperature loading.This conclusion is significant as excellent thermal insulation performance contributes to provide the inner devices of engineering equipment,such as electronic devices and working facilities in a suitable working condition in structure.

Fig.4 Variation of upper temperature with heating temperature for two test conditions.

The increase of inlet velocity facilitates the heat transfer between the lattice structure and cooling air,therefore,leading to the decrease of cooling temperature gradually.Besides,the influence of thermal boundary condition on thermal insulation is also studied,and its effect is only significant at the relatively high heating temperature.This behavior may be attributed to the fact that the heat originated from heating surface cannot be removed efficiently by cooling air at high heating temperature,indicated by the rapid climb of upper temperature in Fig.4.In this situation,the boundary condition plays a certain role in enhancing the thermal dissipation.

Table 2 presents the measured airflow velocity at the inlet section and temperature distribution on cooling surface for Case C.The actual airflow velocity for the two airflow orientations exists a little different and that is acceptable for the forced convective experiment.Overall,the upper temperature decreases with the increase of inlet velocity,while the temperature increases gradually along the flow direction,as indicated by the temperature of N1,N5 and N3 in orientation OA,and the temperature of N1,N5 and N2 in orientation OB.Similar to the previous two cases,the orientation OB in Case C exhibits the lower mean temperature than orientation OA within the velocity range considered, demonstrating the excellent thermal insulation performance.Next,the heat transfer performance for two airflow directions is compared by numerical simulation due to the limited experimental data.

3.Numerical simulation

3.1.Numerical model

In light of the absence of measuring data for heat flux applied on heating surface in experiment,the heat dissipation performance could not be obtained directly.Numerical simulation is carried out to compare the heat transfer performance for the two flow orientations.Furthermore,the numerical method can access into the local thermo-fluidic features which are critical to reveal the heat transfer mechanism induced by the lattice core.

Table 2 Force convection experimental results for Case C.

The numerical model of Kagome lattice sandwich panel for orientation OA and OB are schematically illustrated in Fig.5.They are labelled as ‘‘Kagome-OA” and ‘‘Kagome-OB”,respectively.Given the geometrical periodicity of the lattice core,only two arrays of cells along streamwise direction are considered with the purpose of numerical stability and calculation efficiency.In order to draw a comparison with the temperature measured in experiments, the position of the thermocouples N1 and N3 are included in numerical model for Kagome-OA.Similarly,the thermocouples N1 and N2 are contained in numerical model for Kagome-OB.Besides,the center position is selected and marked as N5′to compare with the thermocouple N5 since they have the same distance to the inlet section.The thermo-physical properties for solid and fluid domains are summarized in Table 3.Given that only the variation of specific heat for air has a significant effect on heat exchange under the steady state condition,its value is various from 1004 J/kg·K to 1022 J/kg·K within the operating temperature range of 0-100°C.

The experimental airflow velocities with a constant static temperature of 300 K are adopted as the inlet boundary condition.With consideration of robustness and numerical stability,the average static pressure is specified at the outlet section.For the interface between solid and fluid domains,conservative interface flux conditions are adopted for heat transfer incorporating General Grid Interface(GGI)mesh connection.The other outer surfaces are set to be adiabatic.Especially,translational periodic boundary condition is applied upon the two side surfaces.To this end,the property of hard link for the two side surfaces is set to ensure the coincident mesh is generated.

Table 3 Thermo-physical properties of titanium alloy and air.

Considering that the heat flux applied on heating surface is not measured directly in force convective experiment,its value is derived based on the measured heating temperature.Firstly,the measured average heating temperature is applied in numerical model to obtain the initial heat flux.Then the heat flux is adjusted and determined until the calculated average heating temperate is identical with the experimental value.As the heating temperature reports the heat input through the sandwich panel,hence,the calculated heat flux eliminates the uncertainty of heat loss in the experiment.

3.2.Validation of numerical model

A hybrid mesh incorporating both tetrahedron and prism grids are generated for the fluid and solid domains by ICEM CFD 17.0,see Fig.5.To well resolve the flow and thermal boundary,fine mesh with ten layers of prism elements in fluid domain is generated near the interface,while relatively coarse mesh is adopted for the rest computational domain to reduce computational cost.Overall,high mesh densities with approximately 2.7×107and 3.0×107elements are distributed in numerical model Kagome-OA and Kagome-OB respectively to reduce the influence of mesh dependency.

Fig.5 Numerical model and representative mesh of Kagome lattice for two orientations.

The conjugated heat transfer problem is solved by the double precision solver ANSYS FLUENT 17.0 based on finite volume method and the time-marching algorithm.To incorporate the inner turbulence effect,the Shear Stress Transport(SST)turbulence is adopted which behaves well for complex flow separation in the range of high Reynolds number.32,33It has been demonstrated that the turbulence model achieves a reasonable agreement between the numerical result and experimental data for lattice structure in forced air convection.3,34Besides,high resolution scheme is selected to discretize the momentum and energy equations.The solution is thought to be converged when the normalized residuals of all the governing equations are less than 10-5.

Before a reliable comparison for the two flow directions,validation of the present numerical model is carried out.Fig.6 first presents the distribution of dimensionless distance(Y+)for two topology orientations at the highest Reynolds number of 13000.Overall,it is obvious that the Y+for both the topology orientations is less than 1.0,which demonstrates the size of first-layer mesh in the wall region satisfies the requirement of wall functions.Thus,the meshes and SST turbulent model are suitable to perform the subsequent simulations.

Table 4 presents the numerically predicted temperature distribution on the upper facesheet and the maximum deviation with experimental data under identical inlet velocity.It can be seen that the variation of temperature obtained numerically and experimentally agrees reasonably well with each other with a maximum deviation of 10.91%.For the Kagome-OA,the measured profile is excellently predicted with a maximum deviation less than 6.5%.In addition,the temperature distribution in the previous two cases is also compared and a reason agreement between numerical simulation and experimental measurement is achieved. Hence, the numerical model is believed to be suitable for classifying the heat transfer characteristics for the two airflow orientations.

4.Numerical results and discussion

4.1.Overall heat transfer performance

To evaluate the heat transfer performance and flow resistance of the truss-cored Kagome-OA and Kagome-OB over a wide range of airflow velocity, three dimensionless parameters,including the Reynolds number ReH,Nusselt number NuHand the pressure drop coefficient fH,all choosing the unit cell height(h)as the characteristic length,are defined as follows,

where ρf,μfand kfare the density,the dynamic viscosity and the thermal conductivity of air,respectively.Umis inlet velocity,ΔP refers to the pressure drop and dpis the heat transfer coefficient,defined as,

where Aupis the area of the heating surface.For the isotemperature boundary condition,Q is the heat flux the temperature difference ΔT is defined as,8

where Tin,Toutand Tware temperature at the inlet,outlet and bottom endwall.

Fig.6 Dimensionless distance for interfaces at the Reynolds number of 13000.

Table 4 Predicted upper temperature distribution and maximal deviation with the experimental data.

Fig.7 Comparison of overall Nusselt number as a function of Reynolds number for two airflow orientations.

Fig.7 compares the overall heat transfer performance for Kagome-OA and Kagome-OB subjected to forced air convection.Within the Reynolds number range of 4330-13000,a significant heat transfer enhancement is achieved for Kagome-OA at the low Reynolds number,while becomes unobvious with the increase of Reynolds number.Overall,the Nusselt number of Kagome-OA is approximately 2%-20%higher than that of Kagome-OB due to the unique airflow direction.Further,the numerical results reveal the applied heat flux for orientation OB is less than that for orientation OA.Thus,lower mean temperature is found for orientation OB in experiment than orientation OA even if the capacity of heat removal for the latter is more effective than that for the former.Since the flow blockage19of orientation OA,0.16 is larger than that of orientation OB,0.12,the difference in heat dissipation for two directions may be attributed to the more serious flow stagnation of Kagome-OA,as pointed out by Joo.19Hench,a detailed description of the fluid flow behaviors is performed in next section.

4.2.Comparison of fluid flow and heat transfer characteristics

To explore the underlying heat transfer enhancement mechanisms,the thermo-fluidic characteristics for the two flow directions in Kagome lattice sandwich panel have been classified.

Fig.8 depicts the fluid flow characteristics of Kagome-OB,illustrated by streamlines and contours of velocity magnitude.Overall,the flow in Kagome lattice sandwich panel is generally straight and parallel to the endwalls when air flows along orientation OB,as shown in Fig.8(a).Owing to the existence of trusses and vertices,the phenomenon of separation and stagnation appears around the trusses,leading to a certain irregularity of primary flow.Especially,the vertices of truss L1 induce complex vortical flow patterns.Behind the vertex of truss L1 intersected with the upper endwall,a clockwise vortex(viewed from above)is induced by the strong shear from primary flow,then becomes skewed spiraling towards the center vertex(Fig.8(b)).Similarly,when the fluid flows across the center vertex,two secondary flows are induced due to the separation of upstream fluid,and form the legs of a pair of counter-rotating vortices,as shown in Fig.8(c).These vortices around each vertex exert strong shear to the surfaces,which no doubt enhances the local thermal dissipation.

Fig.8 Fluid flow characteristic of Kagome-OB.

Fig.9 Spiral primary flow of Kagome-OA.

However,due to the unique topology orientation when air flows along direction OA,the Kagome-OA lattice induces two large scale spiral primary flows within one array of unit cells,which are symmetric with the z-x plane through center vertex as a symmetric plane(denoted as‘‘middle plane”).One of primary flow in the right half unit cells(viewed from upstream)is highlighted by the streamlines in Fig.9(a).The fluid flow passing through the upstream surface of truss L2 separates into two intertwining flow motions.Above the truss L2,the flow(S1)rotates clockwise towards the sidewall.Similarly,the flow(S2)below the truss L2 forms a clockwise rotation towards the middle plane.Therefore,the two spiral flow motions form a large scale spiral primary flow.In particular,when the flow(S2)impinges onto the upstream surface of truss L2,a strong pressure gradient along z-axis appears within the triangular region.Subjected to the effect of strong pressure gradient,the flow(S2)rolls up and becomes skewed towards the inclined truss L2,which intensifies the large scale spiral primary flow.

To further classify the formation of spiral primary flows,three parallel y-z planes shown in Fig.9(a)are selected,and the tangential velocity vectors with velocity contour on corresponding plane is shown in Fig.9(b).It is seen clearly that two symmetrical spiral primary flows form indicated by the strong tangential flow motion,with the middle plane as a symmetric plane.At the center of each spiral primary flow,the fluid velocity is relatively high as it's not blocked by the trusses.Meanwhile,within the region near the endwalls and middle plane,the low velocity flow is dominated.Such a velocity gradient between the center and the periphery of primary flow provides the another contributor for flow rotation.Therefore,the spiral flow motion becomes more evident with the development of flow mixing.

From the different fluid flow behaviors for the two topology directions, it can be concluded that although the Kagome-OB lattice induces complex local vortices,the primary flow is generally parallel to the endwalls.However,the Kagome-OA lattice forms two large scale spiral primary flows in an array of unit cells,which no doubt can greatly improve the heat exchange for endwalls and lattice core. Next,Fig.10 presents the endwall heat transfer patterns in term of local Nusselt number which is based on the temperature difference ΔTdefined in Eq.(3).It can be seen that the entry effect is obvious but rather limited before flowing through the first unit cell.Overall,the local heat transfer near the vertical regions is enhanced corresponding to the complex flow separation and stagnation.However,an evident difference in the heat transfer distribution for the two airflow orientations has emerged caused by the different fluid flow behaviors. For the Kagome-OA,local Nusselt number on the endwall gradually increases along the streamwise direction,as shown in Fig.10(a).The increase may be attributed to the fact revealed in Fig.9 that the spiral flow motion becomes more evident along streamwise direction,which results in the enhancement of heat exchange between endwall and cooling air.As to Kagome-OB,the local Nusselt number exhibits a gradual decrease along streamwise direction due to the fully developed thermal flow.18For a given Reynolds number of 6935,the large scale spiral primary flow and the induced complex flow mixing in Kagome-OA lead to an area-averaged Nusselt number 9.7%higher than Kagome-OB,which is the first contributor to the overall heat transfer enhancement.Furthermore,it is expected that the enhancement may become more pronounced as the thermal conductivity of lattice sandwich panel increases.

Fig.10 Heat transfer distribution on endwall for.

Subsequently,the local heat transfer distribution on the lattice core in the fifth unit cell is displayed in Fig.11.The local Nusselt number is also based on the temperature difference of Eq.(3).On the upstream surface,local heat transfer is dominated by the fluid flow impinging onto the edge regions,which leads to higher local Nusselt number than that on the downstream surface.However,it is different that the strong shear by the spiral primary flow in Kagome-OA causes the higher local heat exchange than that in Kagome-OB,indicated by the larger region of high Nusselt number.On the downstream surface,local heat transfer is governed by the low-momentum flow,thus implying relative low heat dissipation capability.In particular,the local heat transfer is corresponding to the flow behavior as mentioned previously for the two airflow direction,i.e.,since the truss L2 of Kagome-OA intensify the clockwise rotating primary flow,the heat transfer on downstream surface is promoted as a result of fluid wiping the truss.In general,up to 22.5%higher area-average Nusselt number is achieved by lattice core of Kagome-OA than that of Kagome-OB under the identical surface area and porosity.It can be concluded that the heat transfer promotion of lattice core in Kagome-OA is responsible for the another contributor for the substantial overall heat transfer enhancement.

Table 5 Contribution from endwalls and lattice core to overall heat transfer of the sandwich panel.

Furthermore,Table 5 summarizes the contributions from endwalls and lattice core to overall heat transfer mechanisms,evaluated as the product of area-average Nusselt number and heat transfer area.The endwalls devote more than three quarters of overall heat removal in both the two airflow directions due to the high porosity of truss-cored lattice,while a slight increase in percentage for lattice core in Kagome-OA can be found compared with Kagome-OB since its heat transfer enhancement of 22.5%for lattice core is significant than that of 9.7%for endwalls.

4.3.Pressure drop

Fig.11 Comparison of local heat transfer distribution on lattice core.

Fig.12 Comparison of friction factor as a function of Reynolds number.

The pressure drop characteristics for orientation OA and OB are presented in Fig.12 in terms of the friction factor defined in Eq.(3).Both the orientation OA and OB exhibit similar trend with the variation of Reynolds number.The flow is laminar when ReH＜6000,and in transition from laminar to turbulent regime when 6000＜ReH＜9700;the flow becomes turbulent when ReH＞9700 with an approximately constant friction factor of 0.11.It can be seen that in the turbulent state,the difference of pressure drop for the two orientations is not obvious due to the high porosity of 0.98.However,within ReH＜9700,the influence of topology orientation on pressure drop is evident that the orientation OA has 12%-20%higher friction factor than that of orientation OB.This is corresponding to the overall heat transfer performance shown in Fig.7 where the Kagome-OA performs up to 20%higher Nusselt number than Kagome-OB within the low Reynolds number.More evident flow mixing and larger blockage ratio area is responsible for such higher pressure drop of Kagome-OA.

Fig.13 finally performs the overall Nusselt number as a function of dimensional pumping power (fRe3) obtained according to Figs.7 and 12.Within the relatively low pumping power,Kagome OA gives better thermal performance than Kagome OB due to the excellent heat transfer performance,revealing better thermal efficiency is achieved by the former.However, the advantage is decreased gradually with the increasing pump power,and finally overtook by Kagome OB at a certainly high pumping power since the heat removal capacity of both the Kagome lattices is similar beyond a relatively high Reynolds number(Re ＞12000)as shown in Fig.7.Thus, the Kagome OB exhibits the slightly superior to Kagome OA beyond the pumping power of 2.3×1011.

Fig. 13 Comparison of heat dissipation performance as a function of mom-dimensional pumping power.

5.Conclusions

This paper presents a thorough investigation of thermal insulation and heat transfer performance corresponding to two perpendicular flow directions OA and OB for Kagome trusscored lattice sandwich panel.Three test conditions with the variation of heating temperature,thermal boundary condition and airflow velocity are carried out in forced convective experiments.Further,the thermal-fluidic characteristics are compared by numerical method to reveal the underlying mechanisms of heat transfer enhancement.Conclusions drawn in this study are summarized as follows.

(1)The thermal insulation of orientation OB is better than that of orientation OA,indicated by the lower mean upper temperature under the identical temperature loading.In contrary,the orientation OA exhibits 2%-20%higher heat transfer performance in term of overall Nusselt number than orientation OB within the concerned Reynolds number range.

(2)Although the vertices in orientation OB induce complex local vortices,the primary flow is generally parallel to endwalls.Orientation OA induces the large scale spiral primary flows due to the unique topology direction.Further, the velocity gradient between the center and periphery of primary flow and the pressure gradient caused by the trusses intensify the flow rotation.

(3)For a given Reynolds number,the orientation OA exhibits 9.7%and 22.5%higher area-averaged Nusselt number on endwalls and lattice core than that in orientation OB,which are responsible for the underlying mechanisms for heat transfer enhancement.

(4)Complex flow mixing and larger blockage ratio area in orientation OA leads to 12%-20%higher friction factor than that of orientation OB,especially at low Reynolds number(＜9700).

Acknowledgments

The present work was supported by Shaanxi Science and Technology Project of China(No.2016GY-200)and Aeronautical Science Foundation of China(No.20160953008).