# Optimization of Airflow Organization in Bidirectional Air Supply Data Centers in China
## 4 Airflow Organization Optimization
### Analysis of Thermal Environment Evaluation Metrics  
Three thermal performance metrics--the Return Temperature Index (RTI), Supply Heat Index (SHI), and Rack Cooling Index (RCI)--are commonly employed to evaluate bypass airflow recirculation and mixing characteristics in data centers [7; 31]. Notably, SHI can be applied for both room-level and row-level assessments [32]. Figure 13 presents comparative SHI values for each rack row in the baseline and optimized configurations. The baseline model exhibits SHI values exceeding 0.2 for all rows except AC, indicating poor thermal conditions, with average SHI values of 0.215 and 0.245 for unidirectional and bidirectional supply zones, respectively.  
Among individual optimization strategies, Case1, Case2, and Case3 demonstrate limited improvement in cooling utilization efficiency through partial or full aisle containment, while inadvertently increasing cooling loss during airflow delivery to servers in rows AD and AF, resulting in elevated SHI values. Case4 achieves significant reductions after implementing intake-side blanking panels on unoccupied rack spaces, with all row SHI values decreasing below 0.2--representing 46.9% and 37.7% reductions in bidirectional and unidirectional zones, respectively, compared to the baseline, establishing favorable thermal conditions.  
For combined optimization strategies, Case5 shows regressed performance, with some rows (particularly AH at 0.206) exceeding Case4's values. Both Case6 and Case7 achieve  
Figure 13: Comparison of rack’s SHI across different airflow optimization strategies.  
SHI values below 0.1 across all rows, indicating minimal air mixing and optimized cooling utilization efficiency.  
Table 5 presents a comparative analysis of the overall thermal performance metrics between the baseline data center model and various optimization strategies. RTI < 1 across all configurations, indicating predominant cold air bypass with excessive air conditioning supply. Compared to the baseline model, strategies A and B demonstrate slight reductions in RTI, while Case3 exhibits a 1.78% RTI increase.  
Following intake-side blanking panel implementation on unoccupied rack spaces, Case4, Case5, and Case6 demonstrate RTI reductions of 5.85%, 6.74%, and 3.94%, respectively, effectively mitigating hot air recirculation. Case7 achieves complete thermal separation (RTI = 1) with matched air conditioning and equipment temperature differentials.  
All optimized strategies improve cooling utilization efficiency, with SHI reductions of 1.30% (Case1), 0.43% (Case2), 12.17% (Case3), 42.61% (Case4), 40.43% (Case4), 75.65% (Case6), and 88.26% (Case7) relative to the baseline. Prior to the implementation of rack blanking panels, the efficiency hierarchy becomes fully contained cold aisle containment > cold aisle door-only containment > cold aisle top-only containment. Following the implementation of rack blanking panels, the efficiency hierarchy becomes fully contained cold aisle containment > cold aisle top-only containment > cold aisle door-only containment. Under current operating conditions, Case4, Case5, Case6, and Case7 maintain RTI = 1, confirming compliance with ASHRAE thermal guidelines (inlet temperatures below recommended maximums) without localized hotspot risks.