## 1 Introduction  
The major problems of the energy complex of any country are the transition to alternative renewable energy sources and the reduction in the anthropogenic impact of emissions (carbon oxides, nitrogen, and sulfur) from the combustion of fossil fuels. Despite huge investments in the development of the renewable energy sector according to OWID [1], not a single country in the world has managed to replace even half of the consumed heat and electricity with the domestic needs of the population (excluding industrial enterprises) by renewable energy sources. Therefore, at present, the most promising solution to the problems of replacing traditional fossil fuels and reducing the anthropogenic impact of combustion products is the use of new types of fuels. These fuels are a mixture of low-grade solid natural fuels (coal or peat), water (wastewater), and various energy additives; in particular, waste combustible liquids (for example, waste from oil production and refining). They are widely known as coal-water fuels (CWF) [2] and organic coal-water fuels [3]. Previous studies of CWF and OCMF are aimed at developing a preparation procedure, obtaining an optimal composition [4; 5; 6], studying the mechanisms and characteristics of their ignition and combustion [7; 8], as well as studying anthropogenic emissions of their combustion products [9]. The main problem associated with the development of the optimal composition for coal-water fuels is to obtain such fuel, which would have the coal component as high as possible in order to increase the energy content per unit mass while keeping the viscosity low enough for easy transporting. In [4], waste soot was added to the coal-water slurry for enhancing its stability, increasing the caloric value, and reducing the content of nitrogen oxides and sulfur dioxide. The influence of the coal particle distribution, volume fraction,and coal rank in the fuel composition on its rheological properties; in particular, viscosity, has been studied in [5]. By varying the proportion of coal from 30 to 50% in the composition of the slurry, as well as using various additives, the fuels with improved rheological properties and static stability were obtained [6]. The combustion mechanisms of coal-water slurries in air and a fluidized bed [7], as well as laser heating [8], have been studied experimentally. It has been proven experimentally that CWF and OCOWF combustion demonstrates low content of CO\({}_{2}\), NO\({}_{\rm x}\), and SO\({}_{\rm x}\) in flue gases compared to traditional fossil fuels per unit of released energy [9]. In practice, coal-water and organic coal-water fuels are supposed to be burned while spraying them through nozzles or various types of injectors in the furnaces of power-generating installations. Under such conditions, the ignition and combustion characteristics will be significantly affected not only by the spray mode, which determines the size of the droplets and jets formed, but also by the characteristics of the heating surfaces with which some of the droplets will interact. It should be noted that, at present, not only for promising fuels, such as CWF and OCOWF, but also for traditional liquid fuels, the influence of the characteristics of heating surfaces, in particular the texture configuration on the ignition and combustion characteristics, has not been studied at a level that allows formulating practical recommendations for the texture of heat exchange surfaces of power-generating installations. There are known studies of the influence of roughness of the created thermal barrier coatings for the combustion chambers in compression ignition engines [10] and spark ignition engines [11] on heat and mass transfer and fuel combustion. Such coatings were created by plasma-spraying and anodizing methods and a negative effect was found. The roughness increased heat transfer and slowed down combustion. In order to reduce such a negative effect, it has been proposed to apply a coating only to the piston top surface, excluding the chamber [12].