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We measured the natural convection heat transfer of a heated sphere with a point contact with an insulated sphere, varying the location of the contact point along the flow development. Two different sphere diameters of 15.8 mm and 40 mm were used, which correspond to Ra d 's of 3.35 × 108 and 5.43 × 109. Mass transfer experiments using a copper sulfate-sulfuric acid (CuSO 4 –H 2 SO 4) electroplating technique were performed based on the analogy, which visualized the local natural convection heat transfers by the plating patterns. For d = 15.8 mm, the natural convection heat transfers of the heating sphere were similar to that without the contact point regardless of the location of contact points. The copper plating pattern showed the enhancement of heat transfer near the contact point resulting from the redevelopment of the boundary layer compensating for the reduced heat transfer area. For d = 40 mm, the heat transfer of the heating sphere was weaker than that without the contact point only when the contact point was located at downstream from the separation point, which hindered the redevelopment of the boundary layer. This study provides the experimental evidence for the boundary layer redevelopment near the contact point by visualization, which compensates for the reduction in heat transfer area. • Natural convection heat transfer of a heated sphere with a point contact was measured. • Mass transfer experiments were conducted instead of heat transfer ones based on analogy. • Local heat transfer at the heated sphere were visualized by a copper plating pattern. • Heat transfer reduction by contact point is compensated by boundary layer redevelopment. • The redevelopment was not observed for contact points located above the separation point. [ABSTRACT FROM AUTHOR]

• We performed mixed convection experiments in packed beds by mass transfer system. • Sphere diameter, bed height, Re dh and flow direction were varied to cover laminar and turbulent. • Measured velocities and turbulent productions by the PIV were presented and analyzed. • Influence of vortex generation on mixed convection due to packed bed geometry is analyzed. We performed mixed convection heat transfer experiments in a packed bed, which has rarely been investigated but has practical applications in thermal energy storage (TES) systems using highly buoyant fluid operating at low velocities with additional complexity by packed bed geometry. To achieve high buoyancy, mass transfer experiments using a copper sulfate-sulfuric acid (CuSO 4 -H 2 SO 4) electroplating system were conducted based on the analogy between heat and mass transfers. The sphere diameters (d) ranged from 0.004 m to 0.01 m, which correspond to Ra d of 5.43 × 106 and 8.48 × 107. The cases for bed height-to-sphere diameter ratio (H / d) were 5 to 10. The Re dh varied from 1 to 170. The velocity and shear stress were measured to explore the local flow behaviors using Particle Image Velocimetry (PIV). Characteristics of both laminar and turbulent mixed convection were observed depending on the parameter changes. For small bed height (H / d = 5) with large sphere diameter, the Nusselt number (Nu dh) of buoyancy aided flow increased by up to 12.7 % compared to that of buoyancy aided flow, which is the same as the laminar mixed convection behavior. However, for large bed height (H / d = 10) with small sphere diameter, the Nu dh value of buoyancy opposed flow was higher by up to 17.8 % than that of buoyancy aided flow as the Reynolds number (Re dh) increased beyond 50. Despite laminar conditions of this experiment, turbulent mixed convection behavior was observed in the packed bed due to the influence of vortex and turbulent production. This study revealed the geometry influence of packed beds on the vortex generation, which behaves like turbulence. An empirical correlation was developed for mixed convection heat transfer in the packed bed. [ABSTRACT FROM AUTHOR]

• Natural convection of upper and lower spheres and the mutual influences were investigated. • Vertical and horizontal pitches were varied and diameter covered laminar and turbulent. • 4 Distinct effects appeared: stagnant, preheating, initial velocity and chimney-like effects. • The heat transfer of lower sphere was affected by the existence of the upper sphere. • This was evidenced by the visualization of the local heat transfer by copper plating pattern. Natural convection heat transfer of two heating spheres was investigated varying the horizontal and the vertical pitches. Based on the analogy concept, mass transfer experiments were carried out by adopting a copper sulfate-sulfuric acid (CuSO 4 -H 2 SO 4) electroplating system. The sphere diameter was varied from 6 to 40 mm, which correspond to Gr d of 9.10 × 103–2.70 × 106. The heat transfer of two spheres for both in-line and staggered arrangements under laminar and turbulent flow conditions were analyzed by four distinct effects: stagnant, preheating, initial velocity and chimney-like effects. The heat transfer of upper sphere was either enhanced by the initial velocity effect or impaired by the preheating effect. Notable observations were that the heat transfer of lower sphere was also affected by the stagnant flow and chimney effects due to the existence of the upper sphere. This was evidenced by the visualization of the local heat transfer by copper plating pattern. [ABSTRACT FROM AUTHOR]

• Heat transfers of a single heating sphere in packed bed was investigated varying its position. • For natural convection, position influence on the packed bed heat transfer was not observed. • For forced convection, local heat transfers were affected by axial and radial positions. • The transition to turbulent flow regime occurred at ∼0.06 m/s for all sphere diameters. • Forced convection correlation for the local heat transfer in the packed bed was developed. Natural and forced convection experiments on a single heating sphere in a packed bed were performed by varying the axial and radial positions of the heating sphere. Based on the analogy between heat and mass transfer, mass transfer experiments were conducted using a copper electroplating system. The sphere diameter was varied from 0.004 to 0.010 m, which correspond to the Ra d of 5.43 × 106–8.48 × 107. The Sc was 2,014. In the forced convection experiments, the range of superficial velocity was 0.01 to 0.58 m/s, which correspond the Re d of 43–5,076. For natural convective flow, the influence of sphere position on the heat transfer of single heating sphere in the packed bed was not observed. However, for forced convective flow, the local heat transfer inside the bed was affected by the variation of the sphere position. The heat transfer increased at the downstream layer of bed due to the intensified turbulence level by the porous structure. The heat transfer near wall region was also enhanced due to the high local porosity. Based on the experimental results, we developed the correlation of local heat transfer for forced convection by adding the multiplier to the existing correlation. [ABSTRACT FROM AUTHOR]