Study On Coupled Heat And Moisture Transfer Characteristics Of Hygroscopic Phase Change Walls

The phase change materials(PCMs)can be used in walls to moderate the indoor temperature.Similarly,the hygroscopic materials can be used in walls to moderate the indoor humidity.Therefore,their composite materials can be used in walls to moderate the indoor temperature and humidity simultaneously if these two functional materials are combined together.However,there have been few studies on the combined effects of these two materials.The coupled heat and moisture transfer characteristics of hygroscopic phase change walls(HPCWs)are still not clear.Can it make 1 + 1 > 2 if these two functional materials are combined together? This key problem needs to be addressed.Therefore,in this paper,the mechanism study on coupled heat and moisture transfer characteristics of HPCWs were conducted.Firstly,the common coupled heat and moisture transfer models of single-layer hygroscopic walls with different moisture driving potentials(including moisture content,air moisture content,vapor pressure,vapor density and relative humidity)were derived and compared,so as to guide researchers to select and optimize the coupled heat and moisture transfer model.The results showed that models with different driving potentials can be transformed into each other for the same assumptions.Secondly,a novel model for coupled heat and moisture transfer in hygroscopic walls based on phase change criterion(PCC)was built.The PCC was introduced in both the energy conservation equation and the moisture conservation equation to simplify the model.Compared with the common heat and moisture transfer model,the coupling of the proposed model was reduced,which was conductive to the numerical calculation.The relations between PCC and usual material hygrothermal properties under isothermal and non-isothermal conditions were both derived.These relations enabled solving the problems that the empirical determination of the PCC was difficult and the existing models with a constant empirical value of PCC were inaccurate.The proposed model was validated by analytical and experimental results,which had good agreements.Thirdly,the existing coupled heat and moisture transfer models for hygroscopic walls were improved from the perspectives of parameter optimization,algorithm optimization and parameter characterization.Take cellulose insulation as an example,the effects of relative humidity and temperature on liquid water conductivity,moisture diffusivity were considered and discussed.In addition,the effects of relative humidity,temperature and temperature gradient on the PCC were also evaluated.The Tridiagonal Matrix Algorithm(TDMA)was adopted to solve the transient model for coupled heat and moisture transfer through walls,and the comparisons between TDMA and the most used algorithm(i.e.,iteration method)were conducted.The sensitivity analysis was conducted to capture the effects of time step on calculation accuracy of TDMA.Based on the sensitivity analysis results,a novel solution method called Variable Time Step-TDMA was proposed to improve the TDMA.The numerical results showed that the computing time of the improved TDMA was less than that of the traditional TDMA with the same solution accuracy,and the computing time was reduced by 57.4%.A new moisture transfer characteristic index called the temperature gradient factor(TGF)was proposed.It presents the ratio between the moisture flux due to temperature gradient and the total moisture flux.The PCC was introduced to quantify the significance of the vapor flux relative to the total moisture flux.Based on the improved model and two moisture transfer characteristic indexes(i.e.,TGF and PCC),two series simulations were conducted to investigate the non-isothermal moisture transfer characteristics of cellulose insulation.The results showed that the liquid water transfer could be ignored when the relative humidity in the cellulose insulation was less than 60%.Also,the liquid water transfer due to temperature gradient could be ignored.Finally,based on the improved heat and moisture transfer model for hygroscopic walls,two novel heat and moisture transfer models for HPCWs were presented by integrating effective heat capacity method(EHCM)and enthalpy method(EM).Two different cases were conducted to compare the heat and moisture transfer characteristics of three different gypsum boards containing different mass ratios of the PCM.In addition,the effects of different hygrothermal parameters including sorption capacity,water vapor permeability,latent heat of phase change and thermal conductivity on heat and moisture transfer characteristics of HPCWs were quantitatively analyzed.The numerical results showed that the HPCWs can delay and decay the outdoor temperature and humidity fluctuations simultaneously.Therefore,the HPCWs can moderate the indoor temperature and humidity passively,reducing the building energy consumption.Compared with the hygroscopic wall,the HPCWs could reduce the surface heat flux density and surface moisture flux density.Compared with the PG,the heat flux density and moisture flow density on the interior surface of the CMPCM-5 decreased by 26.8% and 34.1%,while these two values for CMPCM-15 were 29.9%and 28.6% respectively.Therefore,the addition of PCMs into the hygroscopic walls can reduce the surface heat flux density and surface moisture flux density,thus reducing the energy consumption and the condensation risk inside the wall.The combining of PCMs and hygroscopic materials can make 1 + 1 > 2.Also,there exists an optimal mass ratio of PCM to reduce the amplitude of surface moisture flux density to the greatest extent.