The licorice infrared combination drying equipment with a heat pump consists of components such as the drying-box shell, heat-pump unit, moisture exhaust fan, and circulation fan, forming a three-heat-source dryer. Its heat sources include a heat pump, U-shaped heating tube, and carbon fiber infrared heating plate. The heat pump is the main heating device, and the heated air is used as the medium to heat the material. The carbon fiber infrared heating plate can dry the material by electromagnetic radiation, and its penetrating effect can accelerate the drying of the material. The U-shaped heating tube is arranged in the air duct 6, which plays the role of supplementing the heat source during the hot drying of the material. If the air temperature is not high enough or the drying needs to be accelerated, it can be opened to reheat the air flowing into the drying chamber to accelerate the drying of the material. The total structural dimensions of the casing (Figure 1) are 1,000 mm × 650 mm × 1,000 mm, and the chamber is divided into a drying chamber, an airflow duct, and a circulating air compartment by vertical and horizontal partitions. The housing’s front panel features an equipment door and a material bin door, with a human–machine-interaction touch screen positioned above the equipment door on the left side. At the rear of the equipment are the circulating air bin, heat pump unit, U-shaped heating pipes, and control unit. The entire drying chamber, corresponding to the bin door on the right side, is equipped with a material storage unit, carbon-fiber infrared heating plate, and data monitoring unit. The bottom of the box is fitted with universal wheels, facilitating easy maneuverability during loading or unloading.

Upon activation, the drying apparatus initiates its heating mode through a man–machine interactive interface. This process activates the carbon fiber infrared radiation electric heating plate and the heat pump switch, allowing for the adjustment of temperature and humidity within the drying space. Preheating continues until the drying chamber reaches a stable state. Then, the licorice drying begins. The operator places a tray of licorice onto the material tray and secures the bin door. When the heat pump heats the air, the refrigerant is compressed into high temperature and high pressure gas by the compressor flows to the heat exchanger 1 (equivalent to the evaporator), and the heat exchanger 2 is equivalent to the condenser. The external fan pumps the circulating air bin where the compressor is located to the negative pressure state. The air in the drying room passes through the heat exchanger 1 into the circulating air bin and is heated. Under the pumping of the circulating ventilator, hot air is then introduced into the drying chamber via the air flow duct, heating the licorice. Concurrently, the data monitoring unit performs real-time monitoring of temperature and humidity within the chamber. The collected data are transmitted to the control unit, which processes the information and manages the operation of the heat exchange unit. The monitoring data within the drying chamber are displayed in real-time on the human-computer-interaction touch screen, enabling the operator to make informed decisions and adjust modes as necessary. The circulation fan plays a crucial role in enhancing air circulation within the enclosure and facilitating airflow through the ducts, thus improving the efficiency of the heat exchange unit. After the drying process is completed, the apparatus is powered off, and the box door is opened to retrieve the dried licorice.

Figure 3 illustrates the numerical model of the air distribution chamber within the dryer, featuring an air inlet, equalization chamber, and air outlet. Airflow during the drying process initiates at the air inlet, progresses toward the right and downwards, and ultimately exits through the left side. The dimensions of the inlet are 630 mm × 100 mm, and the height of the equalization chamber is 600 mm. The chamber’s air outlets consist of 63 evenly spaced circular outlets, each with a radius of 20 mm and a 65 mm gap between adjacent outlets. These outlets are arranged into seven layers within the drying chamber, identified as drying layers 1 through 7, from top to bottom. Meshing for this model was performed using COMSOL Multiphysics 6.1, employing a coarser mesh to ensure convergence. The total mesh count is approximately 593,647, as depicted in Figure 3.

The flow dynamics of Newtonian fluids are primarily characterized by the Navier–Stokes equation, a fundamental principle governing the behavior of viscous, incompressible fluids is calculated as Equation 6 (20):

where ρ denotes the fluid density, x j denotes the coordinate component along the j -direction in the flow field (m), u j represents the mean relative velocity component along the j -direction in the flow field (m/s), δ represents the Kronecker increment, μ is the dynamic viscosity (kg/ms), and g denotes the acceleration of gravity (m/s²).

For turbulence calculations, the Reynolds-averaged Navier–Stokes equations (RANS) are prevalently used. In this context, a standard k-ε turbulence model, known for its robustness and adaptability to complex flow characteristics, was selected. The iterative solution utilized the SIMPLE algorithm for pressure–velocity coupling, with the relevant Equations 7–10 (21):

where k represents the turbulent kinetic energy (m²/s²), μ denotes the viscosity (Pa∙s), μ T denotes the turbulent kinematic viscosity (m²/s), C ε 1 and C ε 2 represent the turbulence modeling coefficients, σ ε and σ k are the Prandtl numbers for kinetic energy and kinetic energy dissipation, respectively, ε denotes the turbulent kinetic energy dissipation rate (m²/s³), and C μ denotes an empirical constant factor.

To optimize work efficiency while maintaining accuracy and minimizing unnecessary computational efforts, several assumptions were made in this experiment:

The experimental setup began with defining the intake boundary conditions, assuming that the airflow direction was perpendicular to the boundary with a normal inflow velocity of 1.2 m/s. The turbulence intensity was set at a medium level (0.05), and the turbulence length scale was determined by the geometry. Pressure outlet boundary conditions were integrated with actual working conditions to estimate the outlet turbulence intensity based on the Reynolds number and turbulence intensity at approximately 5%. Due to the strong velocity gradients induced by the wall driving the flow, the flow simulation near the solid wall region was adjusted using the standard wall function approach with the no-slip wall boundary condition (22).

The velocity deviation ratio E was employed to assess the variance in inlet air velocity across different drying layers, aiding in analyzing the flow velocity distribution within each layer of the drying chamber. The uniformity of the velocity distribution was determined using the velocity inhomogeneity coefficient M (23), calculated as Equations 11–12:

where V L ¯ represents the overall mean value of inlet air velocity for each drying layer (m/s), V a ¯ represents the overall mean value of drying oven inlet velocity (m/s), σ v denotes the standard deviation, n denotes the point location, and V i denotes the velocity at each point (m/s).

To validate the developed numerical model of the dryer, a comparison with actual values was necessary. Actual values were measured using a hot-wire anemometer (TES-1304, TES Electronics Industry Co., Ltd., Taiwan, China). Analog values were determined using the point probe tool in COMSOL Multiphysics. The drying test was conducted at 60°C. Before measurements, the machine underwent a 30-min warm-up phase, and wind speed measurements were initiated upon achieving operational stability. To ensure data precision, the air velocity probe was placed vertically 20 mm from each air exit. Measurements were stabilized before recording, with three readings per outlet averaged to obtain consistent data.

Figure 4 presents a comparison between actual and simulated wind speed measurements, revealing that the modeled values are generally higher than the measured ones. This difference is likely due to structural and sealing variations within the actual drying box, which lead to wind energy losses not accounted for in the idealized simulation scenario. Despite this, the overall trends of the simulated and measured speeds are similar. Starting from layer 1, the wind speed progressively increases, with the maximum deviation between real and simulated wind speeds in the same drying layer being 9.28%. This deviation is within the acceptable confidence level of 10% (24), affirming the numerical model’s validity for guiding future flow field optimization.

During the initial model validation, it was observed that the velocity in each drying layer increased progressively, contrary to the desired uniform drying conditions. Consequently, optimizing the airflow distribution chamber’s velocity flow field is essential for achieving uniform drying.

Geng et al. (25) suggested that introducing a semi-cylindrical spoiler on the left side of the airflow distribution chamber could improve the uniformity of incoming air velocity across each drying layer. To explore the most effective optimization technique, a preliminary feasibility test was conducted. This involved introducing a baffle plate with four rows of 24 circular baffle holes, each with a radius of 30 mm, positioned 40 mm to the right of the air outlet (as shown in Figure 5). The average velocity of the air inlet for each drying layer was determined using the boundary probe function in COMSOL Multiphysics. Figure 6 display simulated velocity sections and graphical comparisons before and after introducing the screed plate.

The simulation comparison diagram reveals that, prior to installing the even wind plate, a portion of the drying gas enters the air distribution chamber, collides with the inner wall, flows downward, and accumulates at the chamber’s bottom, entering the lower drying layers. This results in excessively high airflow velocity in the lower drying layers, while the top drying layer receives minimal airflow, necessitating modifications to achieve the desired uniform drying conditions.

With the introduction of the air deflector in the drying system, the drying airflow is redirected downward through the leveling perforations, effectively channeling a portion of the airflow into the upper drying layer. This design alteration disrupts the vertical downward airflow, thus preventing it from converging at the bottom of the chamber. After the installation of the spoiler, calculations based on probe data indicate a significant improvement in airflow distribution. The average velocity deviation ratio for each drying layer diminishes from 0.5124 to 0.2565%, and the velocity inhomogeneity coefficient is reduced from 0.5913 to 0.3152.

Despite the uniform air plate’s contribution to flow field homogenization, the airflow in the higher drying layers remains considerably lower than that in the lower layers. Consequently, further optimization of the flow field was pursued by adjusting the dimensions of the screed. The adopted strategy involved maintaining the first row of screed holes unchanged while progressively reducing the size of the subsequent rows, as detailed in Table 1.

Table 2 compiles the data from before the drying system’s optimization, the feasibility tests, and after implementing various optimization schemes. The data indicate that Optimization 2 yields the most favorable outcomes. Calculations based on the probe data show that the optimal average velocity deviation ratio is reduced from 0.5124 to 0.2565%, and the velocity inhomogeneity coefficient decreases from 0.5913 to 0.3152, which aligns with the drying uniformity criterion and greatly improves the issue of uneven airflow distribution. Figure 7 illustrates the improved velocity distribution, where the reduction in baffle hole sizes in the lower layer restricts air inflow to the lower drying layer. This stepwise adjustment in the air distribution structure successfully ensures a more balanced airflow across each drying layer, effectively resolving the issue of uneven airflow distribution.

All the samples of fresh licorice were collected from the Urals planted artificially in Qiemo County, Bazhou, southern Tianshan Mountains, Xinjiang, China. The raw materials of three-year-old licorice were extracted from the underground root section at 40 yuan/kg and stored in a refrigerator at 4°C (9). Before the test, the fine roots of licorice were removed, and the mud and sand carried in the process of harvesting were cleaned. To ensure the reliability of the results and reduce errors, licorice root strips with a similar diameter of 10 ± 2 mm were selected and sliced before drying, with a slice thickness of 4 ± 0.5 mm.

In this experiment, licorice was dried using the infrared combined heat pump drying device before and after optimization. Before each test, the drying equipment needed to be preheated. This involved running the drying equipment for 30 min in advance under the set parameter conditions to achieve a stable working state, after which licorice was introduced to initiate the drying operation.

Equipment used in the experiment included a 5427R high-speed refrigerated centrifuge (Eppendorf Company), an L6-180 ultrasonic cleaning machine (Shanghai Haozhuang Instrument Co., Ltd.), a rotary evaporator (Heidolph), an Ms1602Ts 100,000th electronic analytical balance (METTLER Toledo Instruments Ltd.), and an FW type high-speed universal crusher (Beijing Yongguang Medical Instrument Factory). Other equipment included a weighing electronic balance (accuracy 0.01, JA1003 type; Shanghai Precision Scientific Instrument Co., Ltd.) and a color difference meter (Model CR-400; Japan Minolta).

The drying process involved selecting fresh licorice with uniform thickness, complete root shape, full hand feel, no damage, and no rot. After cleaning, the sorted licorice was cross-cut into licorice tablets with a thickness of 4 ± 0.5 mm. The tablets were then weighed using an electronic balance with an accuracy of 0.01 g. The mass of licorice tablets taken from each plate was 50 g, and they were placed on the material tray.

The specific steps of the drying process are as follows: