补充pn2pipefr候选流量诊断

tests/test_amesim_pnl0001_pipe.py

@@ -39,6 +39,24 @@ class AmesimPnl0001PipeTests(unittest.TestCase):
         self.assertLess(reverse, 0.0)
         self.assertAlmostEqual(abs(forward), abs(reverse), delta=abs(forward) * 0.01)
 
+    def test_pn2pipefr_candidate_flow_follows_pressure_gradient(self) -> None:
+        forward = self.pipe.pn2pipefr_mass_flow(
+            port_1_pressure_pa=15.31e6,
+            port_1_temperature_k=293.15,
+            port_2_pressure_pa=15.3e6,
+            port_2_temperature_k=293.15,
+        )
+        reverse = self.pipe.pn2pipefr_mass_flow(
+            port_1_pressure_pa=15.29e6,
+            port_1_temperature_k=293.15,
+            port_2_pressure_pa=15.3e6,
+            port_2_temperature_k=293.15,
+        )
+
+        self.assertGreater(forward, 0.0)
+        self.assertLess(reverse, 0.0)
+        self.assertAlmostEqual(abs(forward), abs(reverse), delta=abs(forward) * 0.01)
+
     def test_calibrated_linear_conductance_overrides_darcy_flow(self) -> None:
         pipe = AmesimPnl0001Pipe(
             name="linear",