Merge pull request MassimoCimmino#301 from MassimoCimmino/issue282_negativeMassFlow

Issue282 negative mass flow

Author: MassimoCimmino

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Version 2.3 (in development)
 
+### New features
+
+* [Issue 282](https://github.com/MassimoCimmino/pygfunction/issues/282) - Enabled the use of negative mass flow rates in `Pipe` and `Network` classes to model reversed flow direction.
+
 ## Version 2.2.3 (2024-07-01)
 
 ### New features

pygfunction/gfunction.py

@@ -1677,8 +1677,10 @@ def solve(self, time, alpha):
                     # The gFunction is equal to the effective borehole wall
                     # temperature
                     # Outlet fluid temperature
-                    T_f_out = T_f_in - 2*pi*self.network.p[0].k_s*H_tot/(
-                        np.sum(self.network.m_flow_network*self.network.cp_f))
+                    T_f_out = T_f_in - 2*pi*self.network.p[0].k_s*H_tot / (
+                        np.sum(
+                            np.abs(self.network.m_flow_network)
+                            * self.network.cp_f))
                     # Average fluid temperature
                     T_f = 0.5*(T_f_in + T_f_out)
                     # Borefield thermal resistance