## 1 Introduction  
Fruits such as apples contain a high amount of water, which causes high metabolic activity, and it will continue after harvesting, turning them into perishable products [1]. Among the methods of preserving food products, drying is considered one of the oldest methods. In the drying process, by removing moisture from the drying product, stability and durability are provided against microbial and enzymatic degradation reactions [2]. The main advantages of drying are the long-term shelf life of products and reducing the costs of packaging, storage, handling, and transportation of products [3; 4; 5]. Today, in the field of dryer manufacturing technology, the third and fourth generations of dryers are available, such as freeze dryers, microwave dryers, infrared dryers, radio frequency dryers, and refractance window dryers, etc. However, still, more than 85% of industrial dryers use convective hot air flow [6; 7]. Despite having good properties, such as simple design, high durability, and low maintenance costs, convective dryers have some disadvantages, such as long drying times, low-quality products, and low energy efficiency [8]. Due to problems, such as high manufacturing cost, complex equipment, slow drying process in freeze dryers, and high moisture content and high sugar content of the final product in osmotic dryers, the third- and fourth-generation dryers were not widely developed [9]. Wide research on the development of convective dryers has been conducted, including investigating the effect of parameters such as drying temperature, air velocity, the humidity of the inlet air, the shape of the airflow, etc. The evaluation of the effect of applying constant and variable temperature by time on the drying characteristics of hemp leaves in a convective dryer showed that the final moisture content of the drying product with variable (increased) temperature rates, compared to drying with constant temperature levels, decreased [10]. The results of the research conducted on the qualitative assessment and drying kinetics of apple slices with an intermittent microwave-convective combined dryer (IMCD) showed that there is a significant difference in the drying kinetics and rehydration in IMCD dryers compared to convective dryers [11]. In research conducted on a convective dryer, the effect of electromagnetic pretreatments and low-humidity inlet air was investigated. According to the results, the combination of infrared pretreatment and inlet air with low humidity can significantly reduce the drying time [12]. Despite the progress in the construction of convective dryers and their widespread applications in industries, their energy efficiency is still low, and drying agricultural products is generally known as an energy-intensive process [13; 14; 15]. Statistics show that a significant amount of energy consumption (about 30%) in the world is related to the agricultural sector, and about 3.62% of this amount is spent solely on drying food products [16]. One of the effective ways to improve energy efficiency and the performance of convective dryers, which has received attention in recent years, is the application of combined dryers. A convective dryer is combined with other dryers, such as infrared, microwave, radio frequency, or other dryers, to improve energy efficiency and speed up the moisture removal rate from the product. The use of infrared waves separately for drying agricultural products was investigated in recent years [17; 18]. Combined hot air-infrared dryers comprise one of these options that have been studied and are still developing. In conducted research on a combined dryer for drying tomato slices, the effects of temperature and infrared radiation on drying time and energy parameters were investigated [8]. In that research, three temperature levels (50, 60, and 70 \({}^{\circ}\)C) and three levels of infrared intensity (0.15, 0.20, and 0.30 W/cm\({}^{2}\)) were used.