adding a tracking code to an order - magento

addTracking.php

    public function addTracking($orderArr){
      $order = Mage::getModel('sales/order')->loadByIncrementId($orderArr["externalordernumber"]);

      $ship = Mage::getModel('sales/service_order', $order)->prepareShipment(count($order->getAllVisibleItems));
      $ship = new Mage_Sales_Model_Order_Shipment_Api();
      $shipmentId = $ship->create($orderArr["externalordernumber"]);

      $shipment_collection = Mage::getResourceModel('sales/order_shipment_collection');
      $shipment_collection->addAttributeToFilter('order_id', $orderArr["externalordernumber"] );

      foreach($shipment_collection as $sc) {
          $shipment = Mage::getModel('sales/order_shipment');
          $shipment->load($sc->getId());
          if($shipment->getId() != '') {
            $track = Mage::getModel('sales/order_shipment_track')
                      ->setNumber('824343454454') //tracking number / awb number
                      ->setCarrierCode('dhl') //carrier code
                      ->setTitle('DHL'); //carrier title
            $shipment->addTrack($track);
        }
      }
    }