Frost Layer Research Articles

Fixed grid numerical modelling of frost growth and densification

A fixed-grid-porous-media method capable of simulating the growth and densification of frost sheets is here presented. A velocity field is calculated across the entire domain, in which a porous media treatment is given to the ice-containing cells. The transported temperature and vapour density are used to define the thermophysical state of each cell, which might enable phase change. As an improvement to Bartrons et al. (2017), the method hereby presented accounts for solidification and sublimation phase transitions. The explicit time step has also been increased by using a semi-implicit treatment of the energy equation. Furthermore, a special boundary condition for cold surfaces has been developed in order to overcome the averaging effect that prevents ice formation in the cells adjacent to the wall. The method is then tested with a study case of a duct flow with a non-homogeneously cooled lower boundary. Several numerical tests are carried out in order to understand the capabilities of the model. The influence of accounting for the convection, as well as the enhanced diffusion resistance factors within the frost layer, is studied by means of the calculated porosity and velocity fields throughout the domain.

Experimental Characterization of Frost Growth on a Horizontal Plate Under Natural Convection

Abstract This paper presents an experimental study on frost formation on a plate under natural convection conditions. Frost thickness, mass, density, and surface roughness are measured during each test. Frost thickness and roughness are measured using an image processing technique. The effect of operating conditions (temperature of the plate, and temperature and relative humidity of the air) on the properties of frost is investigated. Frost surface roughness is quantified using two parameters: the average roughness and the skewness of the roughness, which can describe the frost layer shape. The surface roughness of the frost layer is considerably higher than the roughness of a flat plate, which should be considered in frosting studies. In this paper, it is shown that frost surface roughness is related to the frost layer shape, porosity and density. It is also found that the plate temperature affects the frost surface roughness significantly; as the plate temperature decreases, the frost layer has a high average roughness and negative skewness, which correspond to a highly porous, low density frost layer. Increasing the air humidity and air temperature affects the average surface roughness slightly but not skewness of the frost surface.

Frosting Phenomenon and Frost-Free Technology of Outdoor Air Heat Exchanger for an Air-Source Heat Pump System in China: An Analysis and Review

Frost layer on the outdoor air heat exchanger surface in an air-source heat pump (ASHP) can decrease the system coefficient of performance (COP). Although the common defrosting and anti-frosting methods can improve the COP, the periodic defrosting not only reduces the system energy efficiency but also deteriorates the indoor environment. To solve these problems, it is necessary to clearly understand the frosting phenomenon and to achieve the system frost-free operation. This paper focused firstly on the analyses of frosting pathways and frosting maps. Followed by summarizing the characteristics of frost-free technologies. And then the performances of two types of frost-free ASHP (FFASHP) systems were reviewed, and the exergy and economic analysis of a FFASHP heating system were carried out. Finally, the existing problems related to the FFASHP technologies were proposed. Results show that the existing frosting maps need to be further improved. The FFASHP systems can not only achieve continuous frost-free operation but reduce operating cost. And the total COP of the FFASHP heating system is approximately 30–64% higher than that of the conventional ASHP system under the same frosting conditions. However, the investment cost of the FFASHP system increases, and its reliability also needs further field test in a wider frosting environment. In the future, combined with a new frosting map, the control strategy for the FFASHP system should be optimized.

Computationally efficient modeling strategy for evaporator performance under frost conditions

Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy.

A lattice Boltzmann study of frost growth on a cold surface

A two-dimensional Lattice Boltzmann model was established to study the dynamic physical process of frost growth on a cold flat surface. It is verified that the theoretical results are in good agreement with the experiment data with better accuracy. The frost morphology is explored by showing two-dimension density distribution with different cold surface temperatures. The results also include dynamic characteristics of frost properties such as thickness, deposition weight, volume fraction, density, temperature. According to the results, it is found that the nearer it is from the cold flat plate, the larger of frost crystal volume fraction and density are. The frost surface temperature increases rapidly at the early period of frosting and then increases gradually towards the triple point temperature of the water. The frost internal temperatures rise in a linear tendency with the frost thickness and decrease at the same frost layer location with frost growth. Super-saturation degree of moist air has non-negligible effect on frost growth parameters such as frost thickness, frost crystal deposition mass, frost deposition rate and frost crystal volume fraction.

A study of frost build-up on hydrophilic and hydrophobic surfaces under forced convection conditions

The present study is aimed at investigating, by means of an experimental approach, the effect of surface wettability on the frost accretion over horizontal flat surfaces under forced convection conditions. A purpose-built closed-loop wind-tunnel facility was especially designed and constructed to provide a strict control of the psychrometric conditions at the entrance of the test section, and also of the plate surface temperature. An image acquisition system was used to measure the thickness of the frost layer over time. A dataset comprised of >800 experimental data points spanning different surface temperatures (from −20 to −10 °C), air temperatures (from 5 to 16 °C), modified Jakob numbers (from 1.05 to 2.10), and static contact angles (from 60° to 123°) was gathered and used to investigate both the individual and simultaneous effects of key heat and mass transfer parameters on the frost growth rate. A first-principles modelling approach was used together with the experimental data obtained in-house to come up with a semi-empirical fully-algebraic expression for the frost thickness as a function of the time, the supercooling degree, and the surface contact angle. It was found that the proposed correlation was able to predict most of the experimental data points (>90%) for the frost thickness within ±15% error bounds. Insights on early and delayed nucleation were also obtained from the analysis of the experimental data.

A Numerical Investigation of Frost Growth on Cold Surfaces Based on the Lattice Boltzmann Method

A numerical investigation of frost growth on a cold flat surface was presented based on two-dimensional Lattice Boltzmann model. This model has been validated to have less prediction error by past experiments. According to the results, it is shown that average frost density appears different at an increasing rate at different frosting stages. In addition, cold surface temperature has great influence on frost growth parameters such as frost crystal deposition mass, frost deposition rate, and frost crystal volume fraction. It was found that the frost crystal deposition mass, frost crystal volume, and the deposition rate first increase rapidly, then gradually slow down, finally remaining unchanged while the cold surface temperature decreases. The further away from the cold surface, the more sparser the frost layer structure becomes due to the smaller frost crystal volume fraction.

The effect of surface wettability on frost melting

Frost melting on a vertical surface is divided into three stages: absorption, accumulation and draining, and the effect of surface wettability on melting is investigated analytically. The ratio of the volume flux of water to the melting rate is the key factor that determines meltwater motion. In the absorption stage, the ratio is greater than unity with meltwater is absorbed into the frost layer by capillary force. The volume flux of water depends on porosity and permeability of the frost layer, which are affected by the surface wettability. When the frost layer is saturated, meltwater accumulates on the surface, and the retention water is related to surface wettability. In the draining stage, meltwater flows along the surface, and the draining velocity depends on boundary conditions at the interfaces. Draining velocity increases on hydrophobic surfaces compared to that on hydrophilic surfaces owning to the presence of a slip velocity.

Modeling of frost layer growth considering frost porosity

A numerical model for predicting frost layer growth based on computational fluid dynamics is developed. This model can predict the growth behavior of a highly porous frost layer formed by desublimation. A new volumetric mass transfer rate equation is proposed, which can consider water vapor penetration into a frost layer. The model is validated through experimental results under various operating conditions and used for analyzing the frost layer growth process. The density distribution inside the frost layer is almost linearly changed in the direction perpendicular to the cooling surface under the operating conditions favorable for desublimation, showing different characteristics from the case in the operating conditions favorable for freezing after condensation. In addition, the average mass transfer rate is analyzed as a function of time. As time passes, the porosity of the frost layer decreases and the mass transfer rate due to water vapor penetration decreases gradually.

Frosting and defrosting behavior of slippery surfaces and utilization of mechanical vibration to enhance defrosting performance

We experimentally investigate the frosting and defrosting performance of slippery surfaces which have low sliding angles of water droplets and low ice adhesion strengths. The frosting and defrosting characteristics of slippery surfaces are compared with those of bare aluminum, hydrophilic, and superhydrophobic surfaces. The enhanced sliding properties of water droplets on the slippery surfaces effectively promote the drainage of the condensate on their surfaces, which not only leads to significant frost retardation under frosting conditions, but also substantially reduces the mass of the retained water on the surfaces after defrosting by heat. In addition, when mechanical vibration is applied together with heating during defrosting process, the low ice adhesion strengths of the slippery surfaces enable the effective detachment of the lumped frost layer from their surfaces, thereby significantly reducing the defrosting time.

Experimental and numerical investigations of frost formation on wavy plates

In this work, frost formation on wavy plates with −5 to 2 °C air temperatures, 85% air relative humidity, 0.31–0.92 m/s air velocities and −20 to −5 °C cooling surface temperatures was experimentally and numerically investigated. In the experiments, the frost layer growth process was recorded with frost thickness and weight measured. The experimental results show that the frost thickness increases with time and the frost layer grows faster with higher phase change driving force and higher water vapor mass flux. In the simulations, a frosting model was implemented as the source term in the Euler multi-phase flow method to simulate frosting process. The simulation results of frost layer growth show that densest frost appears at the leading edge of the plate. The frost density on the peaks of the plate is larger than that on the bottom of the valley. The simulations show that the air velocity increases with the frost layer growing. The simulated frost profile, frost weight and frost thickness agree well with the experimental results, which means the frosting model can be used to predict frost formation on wavy plates.

Piezoelectric MEMS resonant dew point meters

This work presents a new class of dew point meters using Thin-film Piezoelectric-on-Silicon (TPoS) resonators. The proposed approach utilizes microelectromechanical resonant balances coupled with a cooling element in which a standard chilled surface technique is used to deposit layers of moisture on the resonator surface. Reducing the temperature causes the resonance frequency of the resonator to increase due to its negative temperature coefficient of frequency (TCF). However, when the temperature of the cooled resonator reaches the dew point (DP) of the surrounding environment, a thin layer of frost starts to form on the resonator surface causing an abrupt negative shift in the resonant frequency due to mass-loading. Therefore, the resonance frequency will be at its maximum value when the dew or frost starts to form i.e., at DP. A custommade setup which includes a MEMS dew point sensor head, a dew point generator and a measurement/control/data collection interface is assembled to enable automated measurements. Using the proposed setup, dew points as low as -41 °C i.e., a dry air containing 145 ppmv H2O, are measured with a standard deviation of 0.1 °C. Long-term reliability and accuracy of the sensors presented here are investigated by conducting 35,000 cycles of uninterrupted measurements. The long-term stability, precision, and durability of the presented dew point meters make them suitable solutions for a wide variety of industrial applications.

Quantitative measurements of nanoscale thin frost layers using surface plasmon resonance imaging

This study reports the presence of a nanoscale thin frost layer. During the frosting process, the surface plasmon resonance (SPR) imaging method can be used to overcome conventional optical limits and quantify this layer. The research outlined here also provides quantitative thickness measurement of the thin frost layer via a proposed calibration method based on the measured SPR intensity. The SPR system established in this study consists of a 50 nm gold-coated BK7 cover glass, a prism, a light source, a polarizer, a lens and a filter for the collimated light of a 600 ± 5 nm wavelength, and a CCD camera. The SPR angle of the ice phase is 72°, which corresponds to the ice refractive index of 1.307. The gold-glass specimen is cooled from room temperature (23 ± 1 °C) to −4.0 ± 0.8 °C by using a thermoelectric cooler to maintain the relative humidity of 20 ± 3% (at the room temperature). As a result, it is found that the nanoscale thin frost layer between the frozen condensates exists on the surface. Also, the present study yields the spatial distribution of reflectance that is associated with the frost layer thickness, indicating that the local information about thin frost layer thickness can be obtained through this SPR imaging method. It is found that the SPR imaging method enables successful capture of the depthwise spatial variations of the thin frost layer, showing that the frost layer was grown over time as a result of the de-sublimation of water vapor.

The role of surface wettability on natural convection frosting: Frost growth data and a new correlation for hydrophilic and hydrophobic surfaces

In refrigeration systems, frost accumulation on the evaporator demands periodic defrosting. Chemical coatings (either hydrophilic or hydrophobic) are sometimes used to mitigate frost build-up and/or improve drainage during the defrost cycle. Although several studies have been performed on frost growth behavior, relatively few studies have specifically examined the effect of surface wettability on the frost growth rate. This paper focuses on the influence of environmental operating conditions on the growth of a frost layer on surfaces which span a wide range of contact angles under natural convection conditions. A new semi-empirical correlation which includes the surface contact angle θ and the modified Jakob number Λ as parameters is reported with an average predictive error of 11.7% (N = 930) for all non-zero experimental data. The correlation was developed using multivariable regression analysis and data which span different surface temperatures (−13 °C to −5°C), relative humidities (40–80%), and static contact angles ranging from 45° (hydrophilic) to 160° (hydrophobic).

Three-dimensional microstructure of frost layer measured by using X-ray μCT

Three-dimensional microstructure of frost layer measured by using X-ray μCT

The Experimental Platform of Frosting by Vapor on Cryogenic Surface in Vacuum Environment

This paper introduces an experimental platform for studying frosting on the cryogenic surface in vacuum environment. This platform contains vacuum system, cold surface temperature control system, image acquisition system, data acquisition system and vapor generation device. The cold surface temperature can be varied in the range of -185 to 0°C, the vacuum degree of the chamber can be controlled in the range of 1 pa to 1bar, and the flow rate of water vapour can be controlled accurately. The experimental platform can be used to study the micro-morphology and the thermal conductivity of frost layer.

An experimental study of defrost on treated surfaces: Effect of frost slumping

Experiments of defrost processes are reported for superhydrophilic, plain and superhydrophobic surfaces which are vertically placed. On the superhydrophobic surface, the frost layer falls off as a rigid body. On the superhydrophilic and plain surfaces, the frost melts, and part of the frost layer falls off with the draining meltwater. Defrost time is thus less for the superhydrophobic surface compared to that for superhydrophilic and plain surfaces. Frost slumping conditions are analyzed with a static force balance, and criteria for frost release are presented. Meltwater motions are suggested as the key factor of the defrost mechanism. When the volume flux of meltwater in the frost is greater than the melting rate, the meltwater is absorbed into the frost. When the volume flux of meltwater is less than the melting rate, it accumulates and drains on the surface. Water accumulation favors frost slumping because the adhesive force becomes weak. Frost slumping generally shortens defrost time and improves defrost efficiency based on our measurements.

Defrosting behavior and performance on vertical plate for surfaces of varying wettability

The defrosting behaviors and performances of super-hydrophilic, bare, and super-hydrophobic surfaces were experimentally investigated along different frost layer densities on a vertical plate. The defrosting behavior can be divided into three types based on the size of the water permeation layer. The defrosting behaviors of bare and super-hydrophilic surfaces were similar, whereas the super-hydrophobic surface behaved differently by allowing the frost layer to be easily removed from its surface. Defrosting performance was evaluated based on defrosting time and water retention ratio. Within the low frost layer density range, defrosting time did not depend on surface characteristics. However, as the density of the frost layer increased, defrosting time increased in the bare and super-hydrophilic surfaces, whereas in the super-hydrophobic surface, that time tended to decrease rather than increase. Water retention ratio was highest on the super-hydrophilic surface and lowest on the super-hydrophobic surface in all frost layer densities. Therefore, the defrosting performance of a super-hydrophobic surface was outstanding over a wide range of frost layer densities.

The behavior of frost layer growth under conditions favorable for desublimation

The purpose of this study is to understand the behavior of frost layer growth under conditions favorable for desublimation. The frosting experiments were conducted on a horizontal cooling surface. Condensation did not occur at the initial stage of frosting, and feather-shaped frost crystals were formed on the cooling surface. These frost crystals grew one-dimensionally while maintaining their shapes. In addition, the effects of operating conditions (air temperature, air velocity, air absolute humidity, cooling surface temperature) on frost layer growth under the conditions favorable for desublimation were investigated. As the cooling surface temperature decreased, the increase in the amount of frost was insignificant. Additionally, an increase in air velocity increased the frost density but not the thickness of the frost layer.

Frost layer thickness measurement and calculation: A short review

As a common physical phenomenon, frost deposition is inevitable and always has significant negative effects in many industry fields, such as aerospace, aviation, and HVAC&R, etc. To accurately predict and control a frosting-defrosting cycle, the frost layer thickness should be firstly obtained, which is the most fundamental and important parameter. During the last several decades, frost layer thickness measurement methods and different correlations were widely reported, basing on experimental, semi-empirical, theoretical, and numerical approaches. To provide an overview of the analytical tools for scholars, researchers, product developers, and policy designers, a review and comparative analysis of frost layer thickness studies were conducted. The existing gaps in the frost research are identified and recommendations are made as authors’ outlooks in final conclusions.