Experimental investigation of two-side heat transfer in spacer-filled channels representative of membrane distillation

•Heat transfer in spacer-filled channels was experimentally investigated.•Thermochromic Liquid Crystals and Digital Image Processing were used.•A new test section allowed two-side heat transfer conditions to be achieved.•Several overlapped and woven spacer configurations were studied and compared.•A detailed sensitivity and uncertainty analysis was performed. Heat transfer in spacer-filled plane channels was investigated by using thermochromic liquid crystals and digital image processing. A novel test section allowed the establishment of two-side cooling, closely representative of the conditions existing in real plane or spiral-wound membrane distillation (MD) modules. Several spacer configurations were investigated, differing in design (overlapped or woven filaments), pitch-to-height ratio and orientation with respect to the main flow direction. The Reynolds number Re ranged from 160 to 2500, as is typical in MD. At all flow rates investigated, symmetric two-side cooling provided significantly higher (25–35%) local and mean heat transfer coefficients hh than one-side cooling. The mean heat transfer coefficient increased when the pitch-to-height ratio increased from 2 to 4, and was much higher for woven than for overlapped spacers. Overlapped spacers with a flow attack angle ϕ of 0°-90° with respect to the two layers of spacer filaments exhibited significantly different distributions and mean values of hh on the two sides, the larger values occurring where the flow was orthogonal to the filaments. In all cases, the dependence of hh on Re could not be described by a simple power law.