Study On Air Distribution And Thermal Comfort In Attached Ventilation Scenarios Under The Restricted Feature In Tiny Sleeping Space

Under the fast-paced pace of contemporary life,tiny sleeping spaces such as capsule hotels,sleep boxes,and soft train sleepers have emerged.With the small footprint,convenience and low-carbon advantages,they have been developed rapidly,and have appeared in many countries and are widely loved.However,the thermal comfort and air quality problems brought about by the small size and highly enclosed have gradually become prominent,so the indoor ventilation environment has become one of the reasons that cannot be ignored that restricts the development of tiny sleeping spaces.Compared with a typical bedroom with dimensions of 4m×3m×2.6m,the limited geometric size of the tiny sleeping space（2m×1.4m×2.4m）brings challenges to the creation of a comfortable and healthy indoor sleeping environment for the following three reasons.First,the supply air jet cannot be fully attenuated in a tiny space.Second,the supply air outlet’s installation location is restricted,and the distance between the personnel and the air outlet is relatively close,which is an easy cause to draught rate.Third,the highly enclosed space layout is easy to cause the accumulation of human metabolic pollutants.In response to the above problems,this paper starts from the needs of the environment in the tiny sleeping space,combined with the unique advantages of attached ventilation,and proposed three attached ventilation scenarios:（1）vertical-wall（i.e.,the sidewall opposite to the bed）-attached ventilation;（2）horizontal ceiling-attached ventilation and（3）double side（i.e.,ceiling and one vertical wall）-attached ventilation.And it is expected that ventilation control can create a comfortable sleeping environment for confined sleeping space.In the paper,a typical physical model and an experimental test cabin of the study object were established using a sleeping box at an airport as a prototype.The air distribution characteristics of the proposed attached ventilation scenarios were studied using experimental testing and CFD numerical simulation.Based on clarifying the influence of basic air supply parameters on the law of airflow distribution,the thermal comfort indicators（DR,FSR,modified PMV）and the air quality indicators（CO₂ concentration and age of air）were used to evaluate the ventilation effects.And the key problems found in the study were verified by experiments and the mechanism was explored.Finally,based on the comprehensive performance evaluation,the double side-attached ventilation was selected to carry out the tuyere design’s extended studies.The conclusions are as follows:Comparing the results of experimental testing and numerical simulation showed that the Realizable k-εturbulence model could most accurately predict the attached supply airflow distribution in a tiny confined space.Then used this model to study the characteristics of air flow in tiny spaces under different attached scenarios.The results showed that the attached mode directly determines the distribution structure of the indoor flow field and the air supply velocity has a greater influence on the flow field in the tiny sleeping space than the air supply temperature.To meet the ambient wind speed requirements in the sleep state,the supply air velocity of the vertical wall-attached mode should be less than 2 m/s,the horizontal ceiling-attached needs to be less than 1.3m/s,and the double side-attached needs to be less than 1 m/s.The analysis of thermal comfort indicators indicated that the attached ventilation could effectively alleviate the severe draught rate in confined spaces.The sleeping zone DR in the three attached ventilation scenarios can be less than 20%,and the vertical wall-attached mode can even be less than 10%.Besides,in the sleeping zone,the PMV,a comprehensive indicator of sleep thermal comfort,is in the range of 0.2-0.8,which can create a slightly warm sleeping environment for tiny spaces.The indoor air quality analysis showed that horizontal ceiling-attached ventilation mode and double side-attached mode have a younger age of air distribution in the occupied zone.Still,vertical wall-attached and double side-attached modes are more conducive to removing respiratory pollutants CO₂.The inconsistency between the age of air and the distribution of CO₂,which are indicators of air quality,indicates that the fastest delivery of fresh air to the target zone does not necessarily guarantee the elimination of pollutants in the target zone.Therefore,a breathing simulation experiment and an age of air tracer gas experiment were designed to verify the above phenomenon and explore the mechanism.Analyzing the experimental data using the dimensionless CO₂ concentration CN and the ventilation efficiencyη,the results confirmed the above phenomenon:the double side-attached ventilation has the best ability to remove respiratory pollutants,followed by vertical wall-attached,and horizontal ceiling-attached the worst;the age of air distribution indicates that vertical wall-attached ventilation has the best air distribution performance,followed by horizontal ceiling-attached and double side-attached.It showed that in the tiny sleeping space,the geometrically confined feature would cause the entrainment circulation flow of the attached ventilation,and the local vortex flow will be formed in different places in the room with the different attached modes.Indoor air quality was determined by the air distribution pattern and the location of the pollution source.Finally,the three attached ventilation scenarios’overall performance was sorted by combining multiple indicators,and the results were double side-attached,vertical wall-attached,and horizontal ceiling-attached in descending order.The limited size of the tiny sleeping space makes the position of the exhaust outlet have a more obvious impact on the airflow distribution.Therefore,at the end of this paper,the double side-attached ventilation mode was selected to optimize the position of the exhaust outlet.The results showed that within the scope of this paper,the best exhaust outlet location is on the same side of the supply air device and above the feet.The research conclusions of this paper will help the development and application of attached ventilation theory in confined tiny spaces,and put forward new ideas for indoor ventilation technology in tiny sleeping spaces.