Co-Flowing Ammonia Desorption in a Fractal-Like Branching Heat Exchanger

Heat activated cooling provides an opportunity to recover and utilize wasted heat. In terms of thermal management of electronics, a heat-activated cooling cycle could be used to thermally manage a space such as a central computing facility. A microscale, fractal-like branching flow heat exchanger was designed and used to desorb ammonia from an aqueous ammonia solution. The fractal-like pattern employed in the present study was previously studied for high heat flux single-phase and two-phase boiling flow heat sink applications. For compatibility, the desorber was fabricated in 316 stainless steel. The desorber is compact, approximately 38 mm in diameter and 6.4 mm thick, and lightweight, weighing 20 grams. Heating was accomplished using Paratherm NF oil between 350 and 400 K. The mass fraction of ammonia in the strong solution inlet stream was 0.30 and the temperature was 300 K. Given a range of inlet solution mass flow rates between 0.42 and 0.92 g/s and oil flow rates between 1.67 and 10 g/s, the mass flow rate of vapor generated varied from 0.02 to 0.13 g/s. The mass fraction of ammonia in the exiting vapor stream varied between 0.8 and 0.96 while circulation ratios varied between 3.5 and 20. Heat exchanger performance is presented using LMTD and ε-NTU analyses. Overall heat transfer coefficients ranged from 7500 to 15,000 for the flow rates and driving temperature differences investigated. The configuration of the desorbers is such that the oil stream can be introduced to flow parallel or counter to the ammonia solution stream. The nature of the microchannels is such that desorption occurs in a co-flowing manner, limiting the vapor mass fraction. However, the advantages of the present design are lightweight, compact, modularity and orientation independence.