Optimization of Finned-Tube Heat Exchanger in a Gravity-Assisted Separated Heat Pipe

Finned-tube heat exchanger(FTHE)is often used as an evaporator in commercial products of separated heat pipe(SHP).The working conditions of FTHE in gravity-assisted SHP are significantly different from those working in refrigerators and air conditioners.Although FTHE is widely used in commercial products of SHP,previous research on its characteristics is very limited.In this paper,a mathematical model for a SHP with FTHE as the evaporator and plate heat exchanger as the condenser is established and verified with experiments.Parametric analyses are carried out to investigate the influences of evaporator design parameters:air inlet velocity,number of tube rows,tube diameter,and fin pitch.With the increasing of air velocity,number of tube rows and tube diameter,and the decreasing of fin pitch,the heat transfer rate increases,while the energy efficiency ratio(EER)decreases monotonically.Using the total cost of the ten-year life cycle as the performance index,the structure parameters of the evaporator with a given heat transfer rate are optimized by the method of orthogonal experimental design.It is found that the total cost can differ as large as nearly ten times between groups.Among the three factors investigated,the number of tube rows has a significant impact on the total cost of the evaporator.With more tube rows,the total cost will be less.The impacts of fin pitch and tube diameter are insignificant.These results are of practical importance for the engineering design of FTHE in gravity-assisted SHP.

Numerical Evaluation of Residual Water Content after Freezing during the Lyophilization of Platelets

Pre-freezing is an important stage in freeze-drying processes.For the lyophilization of a cell,freezing not only plays a role for primary dehydration,but it also determines the amount of residual(intracellular or extracellular)water,which in turn can influence the solution properties and the choice of operation parameters.The freezing of human platelets in lyoprotectant solution is theoretically investigated here.A two-parameter model and an Arrhenius expression are used to describe cell membrane permeability and its temperature dependency.It is assumed that the intracellular solution is composed of four components:sodium chloride,trehalose,serum protein and water,while the extracellular solution consists of three components.Non-ideal solution behaviors are predicted using measured data.The concentration of maximally freeze-concentrated solution is estimated on the basis of an assumption of solute hydration.The impacts of lyoprotectant composition and extracellular sub-cooling on intracellular supercooling and residual water content in the cell are analyzed.The values of activation energy of hydraulic permeability at low temperatures are tested to study their impact on the critical cooling rate.As the mass fraction extracellular lyoprotectant(trehalose+bovineserum albumin)increases from 5 wt%to 20 wt%,the intracellular water content at the end of freezing does not change,but the intracellular solution undergoes much higher super-cooling degree.Increasing the mass ratio of trehalose to bovine serum albumin does not change the intracellular water content,but can mitigate intracellular super-cooling.While 0.05 mol/kg trehalose is loaded into platelet,the total quantity of residual water at the end of freezing may raise by 4.93%.The inclusion of dimethyl sulfoxide(Me2SO)in protectant may bring negative impacts to the drying stage by increasing the residual water content and lowering the drying temperature.

Numerical simulation and experimental research on heat transfer and flow resistance characteristics of asymmetric plate heat exchangers

The asymmetric plate heat exchanger(APHE)has the possibility of achieving balanced pressure drops on both hot and cold sides for situations with unbalanced flow,which may in turn enhance the heat transfer.In this paper,the single-phase water flow and heat transfer of an APHE consisted of two types of plates are numerically(400≤Re≤12000)and experimentally(400≤Re≤3400)investigated.The numerical model is verified by the experimental results.Simulations are conducted to study the effects of,an asymmetric index proposed to describe the geometry of APHEs.The correlations of the Nusselt number and friction factor in the APHEs are determined by taking and working fluids into account.It is found that an optimal exists where the pressure drops are balanced and the heat transfer area reaches the minimum.The comparison between heat transfer and flow characteristics of the APHEs and the conventional plate heat exchanger(CPHE)is made under various flow rate ratios of the hot side and the cold side and different allowable pressure drops.The situations under which APHE may perform better are identified based on a comprehensive index.