US manufacturing industry fails to recover an estimated 900 trillion BTU of low-temperature waste heat from its processes each year. A grand research challenge has been to develop technology that can be applied to recovery of waste heat from industrial manufacturing processes and vehicle operations. The Oak Ridge National Laboratory (ORNL)/Eaton research team is performing computational research and development to design an innovative, direct-contact heat exchanger (DCHE) technology to deliver low- cost, compact, longer-lifetime, high-efficiency waste heat recovery that is optimized for a low- temperature organic Rankine cycle. ORNL resources and expertise in high-performance computing (HPC) and multiphase flows are being utilized to realize this goal while advancing the fundamental understanding of heat transfer between immiscible phases, under turbulent flow conditions in heat exchangers.

The first step of this project was to develop a verified and validated computational fluid dynamics (CFD) model from the vertical DCHE proposed in Ref. \[1\]. The volume of fluid (VOF) model implemented in STAR-CCM+ simulation platform was used to model three-phase flow, along with source terms for boiling, inter-phase drag, gravity, and surface tension forces, along with a low-Reynolds number model for turbulent flow. Because of the stiffness of the source terms that model the physics, low-order spatial and temporal numerical methods were first used to let the numerical solution develop before switching to high-order schemes for higher accuracy. Tuning of relaxation parameters was also required to ensure convergence of the solvers. The mesh consisted of \~3.1 million hexahedral elements and each run required 600 core hours on an HPC cluster. Numerical solutions showed good agreement with experimental data for all experimental conditions presented in Ref. \[1\].

In a second step, a baseline industry-grade DCHE design was developed by adapting the verified and validated CFD model. The industry-grade design consists of two horizontal columns of different cross- sectional areas ─ joined by a convergent-divergent nozzle in the middle, and relies on a three-phase system with co-current flows for operation. Evaporation of the cold liquid (n-pentane or methanol) by the hot liquid (water) occurs in the first column. The first column is fitted with one inlet each for liquid water and liquid pentane (either liquid methanol). The second column of larger cross-sectional area serves as a gravity-driven phase-separator. Two outlets – a drain for liquid water and a vent for superheated vapor are fitted to the second column, andare horizontally staggered to avoid entrainment of the liquid water by the vapor. This baseline design resulted from several trial-and-error evaluations using two-dimensional and three-dimensional CFD simulations to isolate a valid design. A three-dimensional mesh consisted of 0.5 million elements and required 300 CPU hours. Different inlet mass flow rates (2 – 8 kg/s for liquid water and 0.35 – 1.46 kg/s for liquid pentane) and inlet temperature differences (pentane/methanol inlet temperature assumed at 20 C, and water temperature ranges from 50 to 95 C) were also investigated and efficiency was assessed for each operating permutation using a temperature-based efficiency \[1\]. Preliminary results yield efficiencies as high as 85% for the industrial-grade design using an organic Rankine cycle.


High-Performance Computing to Enable Next-Generation Low-Temperature Waste Heat Recovery

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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