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ABSTRACT
The melting thermal characteristics of a microencapsulated phase change material (MEPCM) plate under constant heat flux are numerically investigated. The effective property relations of the MEPCM plate related to the parameters of base materials are first established and verified by the measured thermal properties of 10 samples. A one-dimensional phase change thermal model based on the enthalpy method is built, and the predicted results are also verified with test data. The temperature profile and phase interface movement in the MEPCM plate are discussed, and the effects of Stefan number, phase change temperature range, particle and core fraction, and additive fraction are also analyzed. Three states of solid, mushy, and fluid phases of phase change material (PCM) core divide the melting process of the MEPCM plate into five stages as follows: Fo ≤ 0.1, 0.1 < Fo ≤ 0.2, 0.2 < Fo ≤ 0.9, 0.9 < Fo ≤ 1.2, and Fo ≥ 1.2. The time point of regular regime for heat transfer in the MEPCM plate is Fo = 0.2. The addition of conductivity additive homogenizes the temperature profile in the MEPCM plate. Under the combined effects of thermal diffusivity and Ste number, an optimum additive fraction exists for the MEPCM plate to achieve the minimum melting completion time or the maximum latent storage efficiency.
Nomenclature
| a | = | thermal diffusivity (m2 · s−1) |
| c | = | specific heat capacity (J · kg−1 · K−1) |
| = | dimensionless heat capacity, c/cc | |
| Fo | = | dimensionless time, Fourier number, Fo = λct/ρcccL2 |
| Fom | = | dimensionless melting completion time |
| f | = | liquid fraction |
| H | = | total enthalpy (kJ · kg−1) |
| h | = | latent enthalpy (kJ · kg−1) |
| h′ | = | sensible enthalpy (kJ · kg−1) |
| L | = | thickness of MEPCM plate (m) |
| q | = | heat flux (W · m−2) |
| Ste | = | Stefan number, ce(Tm − To)/he |
| T | = | temperature (°C) |
| Ta | = | ambient temperature (°C) |
| Tm | = | melting medial temperature (°C) |
| t | = | time (s) |
| tm | = | melting completion time (s) |
| x | = | coordinate at the thickness direction (m) |
| Greek symbols | = | |
| η | = | mass fraction |
| θ | = | dimensionless temperature, (T − Tm)/(Tm − To) |
| Δθ | = | dimensionless phase change temperature range, θf − θs |
| λ | = | thermal conductivity (W · m−1 · K−1) |
| = | dimensionless thermal conductivity, λ/λc | |
| ρ | = | density (kg · m−3) |
| = | dimensionless density, ρ/ρc | |
| ϕ | = | volume fraction |
| ψ | = | dimensionless coordinate, x/L |
| Subscripts | = | |
| a | = | adhesive in MEPCM plate |
| c | = | core of PCM in single MEPCM particle |
| e | = | effective property |
| f | = | fluid phase of PCMs |
| o | = | original value |
| p | = | particle of MEPCM in MEPCM plate |
| s | = | solid phase of PCMs |
| sh | = | shell material in single MEPCM particle |
| t | = | thermal conductivity additive in MEPCM plate |
Nomenclature
| a | = | thermal diffusivity (m2 · s−1) |
| c | = | specific heat capacity (J · kg−1 · K−1) |
| = | dimensionless heat capacity, c/cc | |
| Fo | = | dimensionless time, Fourier number, Fo = λct/ρcccL2 |
| Fom | = | dimensionless melting completion time |
| f | = | liquid fraction |
| H | = | total enthalpy (kJ · kg−1) |
| h | = | latent enthalpy (kJ · kg−1) |
| h′ | = | sensible enthalpy (kJ · kg−1) |
| L | = | thickness of MEPCM plate (m) |
| q | = | heat flux (W · m−2) |
| Ste | = | Stefan number, ce(Tm − To)/he |
| T | = | temperature (°C) |
| Ta | = | ambient temperature (°C) |
| Tm | = | melting medial temperature (°C) |
| t | = | time (s) |
| tm | = | melting completion time (s) |
| x | = | coordinate at the thickness direction (m) |
| Greek symbols | = | |
| η | = | mass fraction |
| θ | = | dimensionless temperature, (T − Tm)/(Tm − To) |
| Δθ | = | dimensionless phase change temperature range, θf − θs |
| λ | = | thermal conductivity (W · m−1 · K−1) |
| = | dimensionless thermal conductivity, λ/λc | |
| ρ | = | density (kg · m−3) |
| = | dimensionless density, ρ/ρc | |
| ϕ | = | volume fraction |
| ψ | = | dimensionless coordinate, x/L |
| Subscripts | = | |
| a | = | adhesive in MEPCM plate |
| c | = | core of PCM in single MEPCM particle |
| e | = | effective property |
| f | = | fluid phase of PCMs |
| o | = | original value |
| p | = | particle of MEPCM in MEPCM plate |
| s | = | solid phase of PCMs |
| sh | = | shell material in single MEPCM particle |
| t | = | thermal conductivity additive in MEPCM plate |